One of the first references I found to the straight path was in this passage from Isaiah in The Old Testament.
The voice of one crying in the wilderness: Prepare the way of the Lord, make his path straight. Every valley shall be filled, and every mountain shall be brought low, and the crooked shall be made straight, and the rough ways shall be made smooth; and all flesh shall see the salvation of God.
Old Testament, Isaiah 40:3
This passage can be interpreted as prophesying the coming of Jesus. However, I found this passage significant because it makes the connection between the straight path and achieving salvation, which I also read as drawing closer to God and gaining enlightenment.