1 Corinthians 15:2

By which also ye are saved - On which your salvation depends; the belief of which is indispensable to your salvation; see the note on Mark 16:16. The apostle thus shows the “importance” of the doctrine. In every respect it demanded their attention. It was that which was first preached among them; that which they had solemnly professed; that by which they had been built up; and that which was connected with their salvation. It does not mean simply that by this they were brought into a salvable state (Clarke, Macknight, Whitby, Bloomfield, etc.), but it means that their hopes of eternal life rested on this; and by this they were then “in fact” saved from the condemnation of sin, and were in the possession of the hope of eternal life.

If ye keep in memory - Margin, as in the Greek, “if ye hold fast.” The idea is, that they were saved by this, or would be, if they faithfully retained or held the doctrine as he delivered it; if they observed it, and still believed it, notwithstanding all the efforts of their enemies, and all the arts of false teaching to wrest it from them. There is a doubt delicately suggested here, whether they did in fact still adhere to his doctrine, or whether they had not abandoned it in part for the opposite.

Unless ye have believed in vain - You will be saved by it, if you adhere to it, unless it shall turn out that it was vain to believe, and that the doctrine was false. That it was “not” false, he proceeds to demonstrate. Unless all your trials, discouragements, and hopes were to no purpose, and all have been the result of imposture; and unless all your profession is false and hollow, you will be saved by this great doctrine which I first preached to you.