A.T. Robertson Commentary 1 Corinthians 15:2

"by which also ye are saved, if ye hold fast the word which I preached unto you, except ye believed in vain." — 1 Corinthians 15:2 (ASV)

In what words I preached it unto you (τιν λογο ευηγγελισαμην υμιν). Almost certainly τις (τιν λογο, locative or instrumental, in or with) here is used like the relative ος as is common in papyri (Moulton, Prolegomena, p. 93f.; Robertson, Grammar, p. 737f.). Even so it is not clear whether the clause depends on γνωριζω like the other relatives, but most likely so.

If we hold it fast (ε κατεχετε). Condition of first class. Paul assumes that they are holding it fast.

Except ye believed in vain (εκτος ε μη εικη επιστευσατε). For εκτος ε μη see on 14:5. Condition of first class, unless in fact ye did believe to no purpose (εικη, old adverb, only in Paul in N.T.). Paul holds this peril over them in their temptation to deny the resurrection.