By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
Ye are saved - rather, 'ye are being saved.'
If ye keep in memory what I preached unto you. Bengal, etc, prefer, 'I declare unto you the Gospel (1 Corinthians 15:1) in what words I preached it unto you.' Paul makes known to them, as if anew, not only the fact of the Gospel, but also with what arguments he preached it. Translate, then, 'If ye hold it fast.' I prefer, 'By which ye are saved, if (as I presume is the case, indicative) ye hold fast (in memory and personal appropriation х katechete (G2722)]) 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.