The first article of faith in Islam presents the belief in the Oneness of God. Iman (or “faith” in Arabic), begins with the belief in Allah, from which all other facets of faith do spring. A Muslim must adhere to and acknowledge the idea that no deity is worthy of his or her worship, love, subservience, hope, and fear, other than Allah. Nothing in existence is worthy of one’s ultimate loyalty and sacrifice or worthy of one lowering his or her head in prostration or humility, with the exception of Allah. The Arabic word Allah means God. Allah is not a different God, nor a new God invented by Prophet Muhammad or by Muslims. Allah is the semantic term for God. Arabic Jews and Arabic Christians also use the term Allah in their Books. Islam is a monotheistic religion, and the belief that only One God should be worshipped is fundamental to the faith. The idea of multiple gods is rejected in Islam, as is highlighted many times throughout the Qur’an.
"They have certainly disbelieved who say, "Allah is the third of three." And there is no god except one God. And if they do not desist from what they are saying, there will surely afflict the disbelievers among them a painful punishment" (Quran 5:73)
In another verse, God expresses His wrath for those that attribute a son to Him; calling it highly inappropriate and atrocious for one to commit this sin.
"And they say, ‘The Most Merciful has taken [for Himself] a son’. You have done an atrocious thing. The heavens almost rupture therefrom and the earth splits open and the mountains collapse in devastation. That they attribute to the Most Merciful a son. And it is not appropriate for the Most Merciful that He should take a son." (Quran 19:88-92)
The most serious sin in Islam is known as shirk, which translates to the concept of ascribing a partner to Allah. The term also encompasses the concept of attributing divine qualities to any other besides Allah. In fact, shirk is the only sin that God does not forgive if a person dies before repenting from it.
The Christian belief that Jesus is the son of God, or God himself, is an example of shirk. The belief in the concept of the Trinity--father, son, and/or holy spirit--is a very serious sin in Islam. Allah states in the Holy Quran that on the Day of Judgement, Prophet Jesus will deny ever asking people to worship him instead of God or along with God:
"And (remember) when Allah will say (on the Day of Resurrection): "O 'Iesa (Jesus), son of Maryam (Mary)! Did you say unto men: 'Worship me and my mother as two gods besides Allah?' " He will say: "Glory be to You! It was not for me to say what I had no right (to say). Had I said such a thing, You would surely have known it. You know what is in my inner-self though I do not know what is in Yours, truly, You, only You, are the All-Knower of all that is hidden and unseen" (Quran 5:116)
God is the One to whom worship is due; He is the Creator, the Provider, the Sustainer of everything. He is the Supreme, the Eternal. God has no father nor mother, no son or daughter, no partner nor equal. He is All-Knowing, All-Seeing, All-Hearing, All-Powerful, All-Merciful. It is He who gives life and causes death, it is He who is Unique in His Names and Attributes.
Everything in this world and everything it contains, the whole universe, including you and I, belong to the One God. We use the phrases, ‘my hand,’ ‘my house,’ ‘my money,’ but in reality, it all belongs to God.
"To Allah belongs whatever is in the heavens and whatever is in the earth. Whether you show what is within yourselves or conceal it, Allah will bring you to account for it. Then He will forgive whom He wills and punish whom He wills, and Allah is over all things competent" (Quran 2:284)