Yea, and we are found false witnesses of God; because we have testified of God that he raised up Christ: whom he raised not up, if so be that the dead rise not. 1 Corinthians 15:15 KJV
4Isn't the context clear? [1Cor 15:12-16 NHEB21] Now if Christ is preached, that he has been raised from the dead, how do some among you say that there is no resurrection of the dead? But if there is no resurrection of the dead, neither has Christ been raised. If Christ has not been raised, then our preaching is in vain, and your faith also is in vain. Yes, we are found false witnesses of God, because we testified about God that he raised up Christ, whom he did not raise up, if it is so that the dead are not raised. For if the dead are not raised, neither has Christ been raised.– Michael16Mar 16 at 16:22
We need to follow through the logic of the argument.
Paul and the other disciples have been witnessing that Christ rose from the dead (vv1-11).
So IF (and only if) Christ did NOT rise from the dead, THEN they would be "false witnesses", having witnessed to something which is not true.
The point is that some of his readers have been saying "there is no resurrection from the dead at all". If they were right, it would follow logically that Christ did not rise from the dead either (v13). Therefore their "no resurrection" theory has the direct effect of calling Paul a liar.
In other words, your quoted verse is a conditional statement. It describes something that WOULD BE true if a previously mentioned condition was fulfilled.
1succinctly explained; God bless you. Upvoted. Mar 16 at 17:13