The gospel itself (Paul's gospel that is; justification by faith) is simple. It's only complicated when one is forced to untangle it from human religious tradition.

Folks have one thing in their minds for years (the wrong thing), and then when the truth comes along, it clashes with their erroneous thinking. People sometimes perceive this as "big trouble" and "a complicated mess." But the only complication is that they are trying to maintain their status-quo thinking in the face of the radical new truth. Something's got to give.