From the beginning of Christianity, during the lifetime of Paul of Tarsus, there exist two different Christian religions bearing the same name but with contradictory foundational beliefs.
On one side was the Christianity of the early Jerusalem community, under the leadership of James the Just, which teaches the belief in the absolute uniqueness of God and in a life in agreement with the laws of the Torah so that the Unique God may be pleased with the believers and reward them with eternal paradise after their deaths.
On the other side was the Christianity of Paul of Tarsus, which teaches that the salvation of human beings from the sin inherited from Adam and Eve is only accessible through the belief in the death and resurrection of Jesus Christ, the heavenly Son of God; there is no other way to attain it.
It has also taught belief in the Trinity since the 4th century, refusing to obey the laws of the Torah.
This Christianity has been modified several times in the past. Its most massive modification happened in the 4th century A.D.