Calvin, who had the highest regard for Luther's rediscovery of the gospel of free justification, insisted that this doctrine was "the main hinge of the Christian religion." The implication of Calvin's use of this metaphor is that, if we should become unhinged at this point, then the whole of the Christian faith is in danger of collapsing. Since justification addresses the most basic religious question, namely, how offending sinners can find favour and acceptance with God, it lies at the heart of the saving message of the gospel.
Cornelis Venema, The Gospel of Free Acceptance in Christ, p. 12
And by implication with the gospel unhinged the doorway is wide open for other gospels to enter the church. Which of course is precisely what happens.