Will God compromise the plan of salvation and let people in who came some other way?

Les teaches that God will not compromise the plan of salvation by letting someone enter by another way. When asked if God might compromise and let this person in, the answer is no way, because God is not a compromiser. God made salvation so simple and plain that no one can slip in a side door and say, "Well, I thought." Les points to Cain and Abel as the typical example: Abel did exactly what God said, believed what God said, and acted on it; Cain rationalized, thinking his own effort would cause God to accept him, but God did not. Cain's problem was that he did not believe what God said. Les says it is the same today: if people connive around the simple gospel, he does not believe God will accept them. He adds that he does not know hearts, but he must go by what the book says, and the book says this is the gospel by which people are saved. Paul delivered what he also received, not from Christ's earthly ministry but from the ascended Lord, with the finished work of the cross behind. Christ died for our sins according to the Scripture, not as an afterthought but in the preordained plan of the ages.