When did Christians deviate from the true Gospel? From the very beginning, while the Apostles were still alive, there were some Christians who were leading and being lead away. This is recorded in the Scriptures. This corruption was further spread by the Council of Nicaea and pushed upon the world by the Roman Catholic church through persecution and bloodshed.

Yes there are a group of preserved Christians who hold the truth and have been throughout history but it's not the mainstream Christianity that you see around you.

You also hold the same corrupt Christian beliefs that helped birth Islam.

fundamental doctrines(e.g., Jesus' death, resurrection, and deity)

This is the problem. You have taken the blasphemous view that Jesus is deity. This teaching is purely the tradition of men and is not only not found in the Scriptures but the Scriptures speak against it.

Jesus is a man, the perfect man. He was born of God and sent by God to our Savior. The Messiah is not God and to worship him as God is to give God's glory to another, breaking God's greatest commandment. Many are blind to this truth.

Islam is not the way, but if you don't understand where you yourself went astray then you cannot help them find the right way, for you are both lost.

True Christianity has one God who is the Father alone, Creator of all and God of Jesus Christ. Jesus Christ is the last Adam, a perfect man who was chosen and sent to reconcile the world back to God. We put our faith in Christ because through Christ we may know the true God, not because he is God.