I’d suggest studying Christ's discussion of the Law and Prophets. This may be the best place to start for anyone who has studied but questions.
Jesus clearly states that the Law and Prophets were written about Him. That has been lost on the majority of us because of the Dispensational Heresy which erases Christ as coming savior/God-man from the text.
To say that the New Testament is an invention of Constantine is intellectually and factually without warrant and is also a mechanism to erase the divinity of Christ.
The New Testament is a continued revelation of Christ, incarnate God, the One promised to come to take away sin and bring life. There is no life outside of Christ. He and He alone is the narrow door that you have spoken of.
