1 Corinthians 15:2

By which also ye are saved
It was the means of their salvation, and had been made the power of God unto salvation to them. Salvation is inseparably connected with true faith in Christ as a Saviour, and with a hearty belief of his resurrection from the dead, which is the earnest and pledge of the resurrection of the saints; and because of the certainty of it in the promise of God, through the obedience and death of Christ, and in the faith and hope of believers, which are sure and certain things, they are said to be saved already. To which the apostle puts in the following provisos and exceptions; the one is,

if ye keep in memory what I preached unto you;
or rather, "if ye hold fast, or retain"; that is, by faith, the doctrine preached to you, and received by you, particularly the doctrine of the resurrection of the dead; for the salvation that is connected with it does not depend upon the strength of the memory, but upon the truth and steadfastness of faith: it is the man that perseveres in the faith and doctrine of Christ that shall be saved; and everyone that has truly believed in Christ, and cordially embraced his Gospel, shall hold on, and out to the end; though the faith of nominal believers may be overthrown by such men, as Hymenaeus and Philetus, who asserted, that the resurrection was past already; but so shall not the faith of real believers, because the foundation on which they are built stands sure, and the Lord has perfect knowledge of them, and will keep and save them. The other exception is,

unless ye have believed in vain:
not that true faith can be in vain; for that is the faith of God's elect, the gift of his grace, the operation of his Spirit; Christ is the author and finisher of it, and will never suffer it to fail; it will certainly issue in everlasting salvation: but then as the word may be heard in vain, as it is by such who are compared to the wayside, and to the thorny and rocky ground; and as the Gospel of the grace of God may be received in vain; so a mere historical faith may be in vain; this a man may have, and not the grace of God, and so be nothing; with this he may believe for a while, and then drop it: and since each of these might possibly be the case of some in this church, the apostle puts in these exceptions, in order to awaken the attention of them all to this important doctrine he was reminding them of.