What is the most serious threat to the gospel?
For this question I think J.C. Ryle provides us with enduring discernment:
“You may spoil the gospel by substitution. You have only to withdraw from the eyes of the sinner the grand object which the Bible proposes to faith--Jesus Christ--and to substitute another object in His place… and the mischief is done.
“You may spoil the gospel by addition. You have only to add to Christ, the grand object of faith, some other objects as equally worthy of honor, and the mischief is done.
“You may spoil the gospel by disproportion. You have only to attach an exaggerated importance to the secondary things of Christianity, and a diminished importance to the first things, and the mischief is done.
“Lastly, but not least, you may completely spoil the gospel by confused and contradictory directions… Confused and disorderly statements about Christianity are almost as bad as no statement at all. Religion of this sort is not evangelical.”
(taken from a post C.J. Mahaney made on the Together for the Gospel site)