CLAIM: Paul says that salvation is conditional on following Christ (“if you hold fast the word”). He also questions whether they were even Christians in the first place (“unless you believed in vain”). Doesn’t this threaten the doctrine of eternal security?
RESPONSE: When Paul says that they are “saved,” this is in the sense of sanctification—not justification. The grammar for “saved” is in the present continuous (“you are being saved”). The Bible uses the terms “salvation” or “saved” in multiple senses (c.f. Phil. 2:12-13). Here, Paul might be thinking in terms of sanctification. That is, if these believers do not believe in the resurrection, then this will have an adverse effect on their spiritual growth. In this way, Paul isn’t worried that they’ll be saved; he’s worried that they are not being saved.
Under this view, when Paul says “vain,” he means that they are not bearing fruit. The Greek word for “vain” (eike) is used in this sense later in the same chapter. In verse 10, Paul writes, “His grace toward me did not prove vain; but I labored even more than all of them.” In verse 14, he writes, “If Christ has not been raised, then our preaching is vain, your faith also is vain.” In verse 58, he writes, “Be steadfast, immovable, always abounding in the work of the Lord, knowing that your toil is not in vain in the Lord.” Therefore, when Paul uses the word “vain,” he is thinking about the opposite of hard work.
Another way to understand this passage is that Paul is assuming that they will continue to hold fast. The Greek in this passage is a first-class conditional clause. Therefore, the passage can be translated as “Since you hold fast the word…” or “If and I’m assuming for the sake of argument you will hold fast the word…” These types of clauses in the original Greek assume the truth of the prior statement.
 Morris, L. (1985). 1 Corinthians: an introduction and commentary (Vol. 7, p. 197). Downers Grove, IL: InterVarsity Press.