Wisdom Behind Distortion of the Gospel
Question

Why did Allah, may He be glorified and exalted, allow the Gospel to be distorted when He was able to preserve it?
What are the teachings that the Muslims followed before the coming of the Prophet Muhammad (peace and blessings of Allah be upon him)?
Praise be to Allah.Firstly: 
Allah delegated the preservation of the Torah and Gospel to their scholars and monks, based on the evidence of the verse (interpretation of the meaning): 
“Verily, We did send down the Tawrat, therein was guidance and light, by which the prophets, who submitted themselves to Allah’s Will, judged for the Jews. And the rabbis and the priests [too judged for the Jews by the Tawrat after those prophets], for to them was entrusted the protection of Allah’s Book, and they were witnesses thereto.” [5:44]. 
Allah did not guarantee to preserve them as He guaranteed to preserve the Quran. There are a number of reasons for this: 
1. Allah wanted the Quran to remain the eternal Book and the law that would abide until the Day of Resurrection. Allah says (interpretation of the meaning): 
“And We have sent down to you (O Muhammad (peace and blessings of Allah be upon him)) the Book (this Quran) in truth, confirming the Scripture that came before it and Muhaymin (trustworthy in highness and a witness) over it (old Scriptures).” [5:48]. 
There was no need for the previous Books to be preserved, especially since the time of the Quran was close to the time of the Gospel, and there were only six hundred years between them. 
2. That it was to be a test for those who had been given the Book – would they play their role in preserving the Scripture? Would they believe in what it said? Would they follow the Messenger, the Unlettered Prophet, whom they found mentioned in the Torah and Gospel? Or would they persist in their stubbornness and distort, conceal and falsify? 
3. This was also a test for all the followers of Christianity until the Day of Resurrection. They can see that the Book in which they believe is not free of distortions, doubts and loss, and they can see that the Book of the Final Messenger Muhammad (peace and blessings of Allah be upon him) has been preserved and transmitted by so many from so many that no one can doubt its authenticity, so that calls them to believe in the clear Book, the Noble Quran. 
Secondly: 
During the Jahiliyyah (days of ignorance), before the coming of the Prophet (peace and blessings of Allah be upon him), the people were polytheists (mushrikeen) and idol-worshippers. Most of them had no real religion or decent laws, except a few who followed the way of the Messiah (peace be upon him), such as Waraqah ibn Nawfal, and a few Haneefs who followed the religion of Ibrahim (peace be upon him). These few people shunned shirk, idols, alcohol and immorality, and prostrated to Allah alone, the Lord of the Worlds, such as Zayd ibn ‘Amr ibn Nufayl, of whom it is narrated in a saheeh (authentic) report in al-Bukhari that he said: “I will not eat of that which you slaughter on your stone altars, and I will not eat anything but that over which the name of Allah has been mentioned.” And he also used to say: “O Quraysh, by Allah, there is no one among you who is following the religion of Ibrahim (peace be upon him) except me.” He used to try to stop people burying their daughters alive, and he would say to a man who wanted to kill his daughter: Do not kill her; I will sponsor her. And he would take her and when she grew up he would say to her father: If you wish, I will give her to you, and if you wish, I will sponsor her. [al-Bukhaari].
And Allah knows best.