How the Christians strayed: The Messenger of Allah (ﷺ) said, “Do not exaggerate in praising me as the Christians have exaggerated the praise of Jesus son of Mary, for I am only a servant, so say, ‘He is the servant of Allah and His Messenger.’” (Al-Bukhārī, no. 3445)
The Bible states the First Commandment as, “I am the Lord your God, who brought you out of the land of Egypt, out of the land of slavery. You shall have no other gods besides Me.” (Exodus 20:2-3) This is the God of Moses (peace be upon him). It was He who sent Jesus as His servant and Messenger, and not as a god to be worshipped.
In the New Testament, the First Commandment is reiterated: “Jesus declared, ‘Love the Lord your God with all your heart and with all your soul and with all your mind.’ This is the first and greatest commandment.” (Matthew 22:37-38) So Jesus (peace be upon himself) confirms the Lord of Moses, who is the True God worthy of worship and love.
Many Christian theologians and scholars recognise that only one God, the Lord and Creator of all existence must be worshipped. The German theologian Martin Luther, a seminal figure in the Protestant Reformation (died 1546CE) stated: “[Idolatry] consists not merely in erecting an image and worshiping it, but rather in the heart, which stands gaping at something else, and seeks help and consolation from creatures, saints, or devils, and neither cares for God, nor looks to Him for so much good as to believe that He is willing to help, neither believes that whatever good it experiences comes from God.” 
John Calvin, French theologian, pastor and Protestant reformer (died 1564): “We know that when the Israelites worshipped their Baalim (numerous local deities), they did not substitute them in the place of God as to put Him altogether aside, and assign to them the supreme power; nevertheless, this was an intolerable profanation of God’s worship.”  Calvin in his writings reminds the Christians of the warning of Moses to the Israelites against the worship of any other gods besides the Lord of all existence, “You shall not follow other gods, any of the gods of the peoples who surround you, for the Lord your God in the midst of you is a God demanding faithfulness and exclusive worship, otherwise the anger of the Lord your God will be kindled against you, and He will wipe you off the face of the earth.” (Deuteronomy 6:14) Despite this Christians insist on the worship of Jesus son of Mary (peace be upon him). In this, they contradict the Scripture, they contradict their claim to monotheism, and they oppose the teachings of Moses, Jesus and Muhammad (peace be upon them all). Furthermore, the Bible states, “You shall not make for yourself an idol or any likeness of what is in heaven above or on the earth beneath or in the water under the earth. You shall not worship them or serve them for I, the Lord your God, am a jealous God.” (Exodus 20:4) And this forms the basis of the Second Commandment. The Christians make images of Christ, call him “God” and worship him, yet still, claim they are worshippers of the one Lord of creation. According to the Bible, God said to Moses (peace be upon him), “You saw no manner of similitude on the day that the Lord spoke with you out of the midst of the fire.” (Deuteronomy 4:15) The famous English Puritan and theologian Thomas Watson (1686CE) commented, “How can anyone paint the Deity? Can they make an image of that which they never saw? It is impossible to make a picture of the soul, or to paint the angels because they are of a spiritual nature; much less can we paint God by an image, who is an infinite. To worship God by an image is both absurd and unlawful.” (The Ten Commandments, 1692CE)
As Muslims, we believe that Angels are actually a physical creation, but nevertheless, Islam forbids making images and statues of Prophets, Angels or any of God’s creatures that have a soul whether human or beast. Christians seem to have no such qualms. They make images of Prophets, Angels and God yet they have never seen them. Furthermore, they were forbidden from making such depictions in Scripture. What is baffling is that they make statues and images on pure guess-work and conjecture since they do not know what Mary (may God be pleased with her) looked like, nor Jesus Christ or the Disciples (let alone God), yet they make image after image and statue upon statue from their whims. Each country, even each township has its own depictions. In some countries, Jesus is portrayed and drawn as a white European with blonde hair, whilst in others, he is portrayed as being a black African with tight curly hair. If this is not idolatry, then what is idolatry?!
Many Christian scholars affirmed that the true God to be worshipped is the God of all of the Prophets, the sole Lord of creation, yet Satan deceived the Christians and they fell into polytheism by worshipping Jesus and his mother in the form of statues, portraits and the cross or by simply deifying them in their hearts and minds. The Bible states, “You shall not make for yourselves idols, nor shall you set up for yourselves an image or a sacred pillar, nor shall you place a figured stone in your land to bow down to it, for I am the Lord your God.” (Leviticus 26:1) How many places across the world do we see stone images of Mary with baby Jesus in her arms, or statues of Christ himself in churches and elsewhere– some stretching metres high and others small figurines. You see thousands of Christians gathering and prostrating to these images and statues of stone. This is the idolatry that is forbidden in scripture and sound intellect, that nullifies their claim to monotheism.
 The Large Catechism III, Part First: The First Commandment, Translated by F. Bente and W.H.T. Dau Published in Triglot Concordia: The Symbolical Books of the Ev. Lutheran Church, St. Louis: Concordia Publishing House, 1921, pp. 565-773. (ref: wp)
 John Calvin, Harmony of the Law: Part 1, The Law: The First Commandment.