Jamieson, Fausset and Brown on 1 Corinthians 15:2

2. ye are saved--rather, "ye are being saved."

if ye keep in memory what I preached unto you--Able critics, Bengel and others, prefer connecting the words thus, "I declare unto you the Gospel (1Co 15:1) in what words I preached it unto you." Paul reminds them, or rather makes known to them, as if anew, not only the fact of the Gospel, but also with what words, and by what arguments, he preached it to them. Translate in that case, "if ye hold it fast." I prefer arranging as English Version, "By which ye are saved, if ye hold fast (in memory and personal appropriation) with what speech I preached it unto you."

unless--which is impossible, your faith is vain, in resting on Christ's resurrection as an objective reality.