People tend to confuse DESCRIPTIVE passages of Scripture with PRESCRIPTIVE passages of Scripture. 1 Corinthians 15:1,2 - Moreover, brethren, I declare to you the gospel which I preached to you, which also you received and in which you stand, by which also you are saved, if you hold fast that word which I preached to you--unless you believed in vain.
To believe in vain is to believe without cause or without effect, to no purpose. If, as some are saying in Corinth, there is no resurrection, then faith is vain and worthless (vs. 14). The people who fail to hold fast to the word (the gospel) that Paul preached in 1 Corinthians 15:1-4, demonstrated that they "believed in vain" (did not truly believe). Which explains why people fail to hold fast to the gospel/endure to the end.