One of the qualities of believers mentioned in the Qur'an is their belief in the Books of God. As our Lord revealed the Qur'an to our Prophet (pbuh), He also revealed the Gospel to the Prophet Jesus (pbuh), the Torah to the Prophet Moses (pbuh), various texts to the Prophet Abraham (pbuh), and the Psalms to the Prophet David (pbuh). Muslims are obliged to believe in all these Divine books.
God states the following in the verses of the Qur'an:
Those who believe in what has been sent down to you and what was sent down before you, and are certain about the hereafter. (Surat al-Baqara, 4)
Say: 'We believe in God and what has been sent down to us and what was sent down to Abraham and Ishmael and Isaac and Jacob and the Tribes, and what Moses and Jesus were given, and what all the Prophets were given by their Lord. We do not differentiate between any of them. We are Muslims submitted to Him.' (Surat al-Baqara, 136)
A Muslim who abides by the Qur'an believes in all the Prophets and their revelations. Although there are some commandments and statements in these Books which have been distorted over the course of time, they also contain Divine commandments regarding the principles of faith, the way of worship and good morality. The following are some of the verses in the Qur'an that refer to the Gospel:
And We sent Jesus, son of Mary, following in their footsteps, confirming the Torah that came before him. We gave him the Gospel containing guidance and light, confirming the Torah that came before it, and as guidance and admonition for those who have piety. The people of the Gospel should judge by what God sent down in it. Those who do not judge by what God has sent down, such people are deviators. (Surat al-Maida, 46-47)
The Gospel, which confirmed the Torah, was revealed to the Prophet Jesus (pbuh) as guidance and admonition for those who have faith. But after the Prophet Jesus (pbuh) was taken alive to the Sight of God, some parts of the Gospel have been distorted, and this has detracted from its true meaning. The guide that shows which parts of the Gospel is true to its origin and which have been contorted is the Qur'an, the final Holy Book sent by God. The Qur'an confirms the true parts of the Torah and the Gospel and abolishes the altered parts. In this respect, the Qur'an is a judge for those who have faith.
For example, in the Qur'an, God warns Christians about their belief in the trinity and their naming the Prophet Jesus (pbuh) as the son of God, -surely God is above such thoughts- and warns them about their deviant beliefs:
"Those who say that the Messiah, son of Mary, is God are unbelievers. The Messiah said, 'Tribe of Israel! Worship God, my Lord and your Lord. If anyone associates anything with God, God has forbidden him the Garden and his refuge will be the Fire.' The wrongdoers will have no helpers." (Surat al-Ma'ida, 72)
Those who say that God is the third of three are unbelievers. There is no god but One God. If they do not stop saying what they say, a painful punishment will afflict those among them who are unbelievers. (Surat al-Ma'ida, 73)
In Surah Maryam, God informs us how ugly and how big an atrocity the slander "God has a son" is in the following way:
They say, 'The All-Merciful has a son.' They have devised a monstrous thing.
The heavens are all but rent apart and the earth split open and the mountains brought crashing down, at their ascription of a son to the All-Merciful!
It is not fitting for the All-Merciful to have a son. (Surah Maryam, 88-92)
The false belief of the Christians, that "The Prophet Jesus (pbuh) died on the cross", has been corrected in the Qur'an. In the Qur'an, God informs that the Prophet Jesus (pbuh) did not die; people were shown someone who resembled him, and the Prophet Jesus (pbuh) was raised up alive to God's Sight and will return to the world in the End Times, which we are living in today:
And (on account of) their saying, 'We killed the Messiah, Jesus son of Mary, Messenger of God.' They did not kill him and they did not crucify him but it was made to seem so to them. Those who argue about him are in doubt about it. They have no real knowledge of it, just conjecture. But they certainly did not kill him.
God raised him up to Himself. God is Almighty, All-Wise.
There is not one of the People of the Book who will not believe in him before he dies; and on the Day of Resurrection he will be a witness against them. (Surat an-Nisa, 157-159)
While God corrects the Christians' false belief about the so-called 'crucifixion of the Prophet Jesus' (pbuh) with the verses of the Qur'an, He also informs that the true belief about the Prophet Jesus (pbuh) is his 'second return to the world'.
In addition to this wealth of information, both Christians and Jews are defined as the "People of the Book" in the Qur'an, and their food is made lawful to Muslims. Muslims are also allowed to marry women from the People of the Book (Surat al-Ma'ida, 5). This reveals that there should be a very affectionate relationship, a sincere bond of friendship and companionship between Christians, Jews and Muslims.
Indeed, there may be oppressors amongst the People of the Book, as there are in every community. Cruelty is a grave immorality also committed by some whose identity card reads "Muslim". Therefore, true Muslims are obliged to carry out an intellectual struggle to eliminate oppression from the face of the earth. Yet, this struggle of the believers should never be carried out with hatred, animosity, aggression, or violence. Muslims are obliged to convey the errors in the beliefs of the People of the Book "in the kindest way", that is with compassion, mercy, goodness, and friendship. As Muslims, our call to the Christians and Jews according to the commandments in the verses is as follows: "We believe in what has been sent down to us and what was sent down to you. Our God and your God are One and we submit to Him." (Surat al-Ankabut, 46) Our proposition to them is to "believe in One God".
Say: 'People of the Book! Come to a proposition which is the same for us and you – that we should worship none but God and not associate any partners with Him and not take one another as lords besides God.' If they turn away, say, 'Bear witness that we are Muslims.' (Surah Al' Imran, 64)
Only argue with the People of the Book in the kindest way – except in the case of those of them who do wrong – saying, 'We believe in what has been sent down to us and what was sent down to you. Our God and your God are One and we submit to Him.' (Surat al-Ankabut, 46)