At the heart of Islam lies belief in God.
The core of the Islamic creed is bearing witness to the phrase, La illaha illa Allah, “There is no true deity deserving worship but God.” The testimony to this belief, called tawhid, is the axis around which all Islam revolves. Moreover, it is the first of the two testimonies by which a person becomes a Muslim. Striving after the realization of that oneness, or tawhid, is the core of Islamic life.
For many non-Muslims, the term Allah, the Arabic name of God, refers to some distant and strange deity worshipped by the Arabs. Some even think it to be some pagan “moon-god”. However, in Arabic, the word Allah means the One True God. Even, Arabic speaking Jews and Christians refer to the Supreme Being as Allah.
Western philosophers, Eastern mystics as well as today’s scientists attempt to reach God in their own way. Mystics teach of a God who is found through spiritual experiences, a God who is part of the world and resides within His creation. The philosophers seek God though pure reason and often speak of a God as a detached Watch-Maker with no interest in His creation. A group of philosophers teach agnosticism, an ideology that holds that one can neither prove nor disprove God's existence. Practically speaking, an agnostic asserts he must be able to perceive God directly in order to have faith. God has said:
“And those who are devoid of knowledge say: ‘Why does not God speak unto us or why is not a [miraculous] sign shown to us?’ So said the people before them words of similar import. Their hearts are all alike...” (Quran 2:118)
The argument is nothing new; people in the past and present have raised the same objection.
According to Islam, the correct way of finding God is through the preserved teachings of the prophets. Islam maintains that the prophets were sent by God Himself throughout the ages to guide human beings to Him. God says in the Holy Quran that the correct path to belief is to reflect upon His signs, which point to Him:
“…Indeed, We have made all the signs manifest unto people who are endowed with inner certainty.” (Quran 2:118)
Mention of God’s handiwork occurs often in the Quran as the locus of divine revelation. Anyone who sees the natural world in all its wonder with open eyes and an open heart will see the unmistakable signs of the Creator.
“Say: Go all over the earth and behold how [wondrously] He has created [man] in the first instance: and thus, too, will God bring into being your second life – for, verily, God has power to will anything.” (Quran 20:29)
God’s handiwork is also present within the individual:
“And on earth there are signs [of God’s existence, visible] to all who are endowed with inner certainty, just as [there are signs thereof] within your own selves: can you not, then see?” (Quran 51:20-21)
Belief in God in Islam consists of four matters:
(I) Belief in God’s existence.
(II) God is the Supreme Lord.
(III) God Alone is entitled to worship.
(IV) God is known by His Most Beautiful Names and Attributes.
God’s existence does not require proof by scientific, mathematical, or philosophical arguments. His existence is not a ‘discovery’ to be made by the scientific method or a mathematical theorem to be proven. Simply said, mere common sense bears witness to God’s existence. From a ship one learns of the ship-builder, from the cosmos one learns of its Creator. God’s existence is also known by answers to prayers, miracles of prophets and the teaching in all revealed scriptures.
In Islam, a human being is not seen as sinful creature to whom the message of Heaven is sent to heal the wound of original sin, but as a being who still carries his primordial nature (al-fitrah), an imprint on his soul that lies deeply buried under layers of negligence. Humans are not born sinful, but forgetful as God has said:
“…Am I not your Lord? They said: ‘Yes, we bear witness...’” (Quran 7:172)
In this verse, the “they” refers to all human beings, male and female. The ‘yes’ confirms the affirmation of God’s oneness by us in our precosmic state. Islamic doctrine holds that men and women still carry the echo of this ‘yes’ deep down within their souls. The call of Islam is directed to this primordial nature, which uttered ‘yes’ even before they inhabited the earth. Knowledge that this universe has a Creator is something instinctive in Islam and therefore it requires no proof. Scientists, such as Andrew Newberg and Eugene D’Aquili, both affiliated with the University of Pennsylvania and pioneers in the neurological research of religion, say “We are wired for God.” 
The Holy Quran rhetorically asks:
“…Can there be any doubt about God, the Creator of the heavens and the earth?...” (Quran 14:10)
One might ask, ‘if belief in God is natural, then why do some people lack this belief?’ The answer is simple. Every human being has an innate belief in a Creator, but this belief is not a result of learning or personal deductive thinking. With the passage of time, outside influences effect this innate belief and confuse the person. So, one’s environment and upbringing veil the primordial nature from the truth. The Prophet of Islam, may God praise all, said:
“Every child is born in a state of fitrah (a natural belief in God), then his parents make him a Jew, a Christian, or a Magian.” (Saheeh Muslim)
Often these veils are lifted when a human being is faced with a spiritual crisis and left helpless and vulnerable.
God is the only Lord of heaven and earth. He is the Lord of the physical universe and the Lawgiver for human life. He is the Master of the physical world and Ruler of the affairs of men. God is the Lord of every man, woman, and child. Historically, only a few have denied the existence of the Lord, which means that throughout the ages people have, for most part, believed in One God, a Supreme Being, a supernatural Creator. That God is the Lord specifically contains the following meanings:
First, God is the sole Lord and Ruler of the physical world. Lord means He is the Creator, Controller, and Owner of the Kingdom of the heaven and the earth; they belong exclusively to Him. He alone brought existence out of non-existence, and all existence depends on Him for its conservation and continuance. He did not create the universe and leave it to pursue its own course according to fixed laws, thereafter ceasing to take any further interest in it. The power of the Living God is required at every moment to sustain all creatures. Creation has no Lord besides Him.
“Say (O Muhammad): ‘Who provides for you from the sky and the earth? Or who owns hearing and sight? And who brings out the living from the dead and brings out the dead from the living? And who disposes the affairs?’ They will say: ‘God.’ Say: ‘Will you not then be afraid of God’s punishment (for setting up rivals with Him)?’” (Quran 10:31)
He is the ever-ruling King and the Savior, the Loving God, full of wisdom. No one can change His decisions. Angels, prophets, human beings, and the animal and plant kingdoms are under His control.
Second, God is the only Ruler of the affairs of men. God is the supreme Lawgiver,  the Absolute Judge, the Legislator, and He distinguishes right from wrong. Just like the physical world submits to its Lord, human beings must submit to the moral and religious teaching of their Lord, the Lord who sets right apart from wrong for them. In other words, God alone has the authority to make laws, determine acts of worship, decide morals, and set standards of human interaction and behavior. His is the command:
“…Surely, His is the creation and the command; blessed be God, the Lord of the worlds.” (Quran 7:54)
Islam lays much greater emphasis on how belief in God translates into righteous, obedient life and good morals rather than proving His existence through theological intricacies. Hence, the Islamic motto is that the primary message preached by the prophets was surrender to God’s will and His worship and not so much the proof of God’s existence:
“And We never send any Messenger before you (O Muhammad) without having revealed to Him: none has the right to be worshipped but I, therefore you shall worship Me (Alone).” (Quran 21:25)
God has the exclusive right to be worshipped inwardly and outwardly, by one’s heart and limbs. Not only can no one be worshipped apart from Him, absolutely no one else can be worshipped along with Him. He has no partners or associates in worship. Worship, in its comprehensive sense and in all its aspects, is for Him alone.
“There is no true god worthy of worship but He, the Most Merciful, the Most Compassionate.” (Quran 2:163)
God’s right to be worshipped can not be over emphasized. It is the essential meaning of Islam’s testimony of faith: La ilah illa Allah. A person becomes Muslim by testifying to the divine right to worship. It is the crux of Islamic belief in God, even all of Islam. It was the central message of all prophets and messengers sent by God - the message of Abraham, Isaac, Ishmael, Moses, the Hebrew prophets, Jesus, and Muhammad, may the mercy and blessings of God be upon him. For instance, Moses declared:
“Hear, O Israel; The Lord our God is one Lord.” (Deuteronomy 6:4)
Jesus repeated the same message 1500 years later when he said:
“The first of all the commandments is, ‘Hear, O Israel; the Lord our God is one Lord.’” (Mark 12:29)
And reminded Satan:
“Away from me, Satan! For it is written: Worship the Lord your God, and serve Him only.” (Matthew 4:10)
Finally, the call of Muhammad some 600 years after Jesus reverberated across the hills of Mecca:
“And your God is One God: there is no god but He…” (Quran 2:163)
They all declared clearly:
“…Worship God! You have no other god but Him…” (Quran 7:59, 7:65, 7:73, 7:85; 11:50, 11:61, 11:84; 23:23)
Worship in Islam consists of every act, belief, statement, or sentiment of the heart which God approves and loves; everything that brings a person closer to His Creator. It includes ‘external’ worship like the daily ritual prayers, fasting, charity, and pilgrimage as well as ‘internal’ worship such as faith in the six articles of faith, reverence, adoration, love, gratitude, and reliance. God is entitled to worship by the body, soul, and heart, and this worship remains incomplete unless it is done out of four essential elements: reverential fear of God, divine love and adoration, hope in divine reward, and extreme humility.
One of the greatest acts of worship is prayer, invoking the Divine Being for aid. Islam specifies that prayer should only be directed to God. He is deemed in total control of every man’s destiny and able to grant his needs and remove distress. God, in Islam, reserves the right of prayer for Himself:
“And do not invoke, along with God, anything that can neither benefit you nor harm you, for behold, if you do it, you will surely be among the evildoers!” (Quran 10:106)
Giving anyone else - prophets, angels, Jesus, Mary, idols, or nature- a portion of one's worship, which is essentially due only to God, such as prayer, is called Shirk and is the most enormous of sins in Islam. Shirk is the only unforgivable sin if not repented from, and it denies the very purpose of creation.
God is known in Islam by His beautiful Names and Attributes as they appear in revealed Islamic texts without the corruption or denial of their obvious meanings, picturing them, or thinking of them in human terms.
“And the Most Beautiful Names belong to God, so call on Him by them…” (Quran 7:180)
Therefore, it is inappropriate to use First Cause, Author, Substance, Pure Ego, Absolute, Pure Idea, Logical Concept, Unknown, Unconscious, Ego, Idea, or Big Guy as divine Names. They simply lack beauty and that’s not how God has described Himself. Instead, Names of God indicate His majestic beauty and perfection. God does not forget, sleep, or get tired. He is not unjust, and has no son, mother, father, brother, associate, or helper. He was not born, and does not give birth. He stands in need of none as He is perfect. He does not become human to “understand” our suffering. God is The Almighty (al-Qawee), The One Incomparable (al-‘Ahad), The Acceptor of Repentance (al-Tawwaab), The Compassionate (al-Raheem), The Ever-Living (al-Hayy), The All-Sustaining (al-Qayyum), The all-Knowing (al-‘Aleem), The All-Hearing (al-Samee’), The All-Seeing (al-Baseer), The Pardoner (al-‘Afuw), The Helper (al-Naseer), The Healer of the Sick (al-Shaafee).
The two most frequently invoked Names are “The Compassionate” and “The Merciful.” All but one of the chapters of Muslim scripture begins with the phrase, “In the Name of God, the Most-Merciful, the Most Gracious.” The phrase is used, one might say, by Muslims more commonly than the names Father, Son, and Holy Spirit are heard in Christian invocations. Muslims begin in the Name of God and remind themselves of God’s Compassion and Mercy every time they eat, drink, write a letter, or perform anything of importance.
Forgiveness is an important dimension of human relationship with God. Human beings are realized to be weak and prone to sin, but God in His tender mercy is willing to forgive. The Prophet Muhammad said:
“God’s mercy outweighs His wrath.” (Saheeh Al-Bukhari)
Along with the divine names “The Compassionate” and “The Merciful,” the names “The Forgiver” (al-Ghafur), “The Oft-Forgiving” (al-Ghaf-faar), “The Acceptor of Repentance ” (at-Tawwaab) and “The Pardoner” (al-Afuw) are among the most used in Muslim prayers.