I am no expert. But as you know, context determines most things.
1 Cor 15:2~~
New American Standard Bible
by which also you are saved,
if you hold fast the word which I preached to you, unless you believed in vain.
Paul sets His premise on the Gospel of the death and resurrection of Christ And his "word" that he(Paul) preached to the Corinthians. 1 Cor 15:1 is their salvation and they received the "word/Gospel" that Paul taught. And He goes on to reiterate what his "word" was in 1 Cor 15:3-11.
1 Now I make known to you, brethren, the
gospel which I preached to you, which also
you received, in which also you stand,
2 by which also you are saved, if you hold fast the word which I preached to you, unless you believed in vain.
3 For I delivered to you as of first importance what I also received, that Christ died for our sins according to the Scriptures,
4 and that He was buried, and that He was raised on the third day according to the Scriptures,
5 and that He appeared to Cephas, then to the twelve.
6 After that He appeared to more than five hundred brethren at one time, most of whom remain until now, but some have fallen asleep;
7 then He appeared to James, then to all the apostles;
8 and last of all, as to one untimely born, He appeared to me also.
9For I am the least of the apostles, and not fit to be called an apostle, because I persecuted the church of God.
10 But by the grace of God I am what I am, and His grace toward me did not prove vain; but I labored even more than all of them, yet not I, but the grace of God with me.
11 Whether then
it was I or they,
so we preach and so you believed.
"Unless you believed in vain" clues us in. Paul Knows that they received his word. Paul Knows that they believed. 1 Cor 15:11. So Paul knows that they are holding fast the "word/Gospel" which he preached......1 Cor 15:11 shows us this.
He KNOWS they didn't believe in vain....So he knows they hold fast his word. So the context tells us that it is a first class conditional clause.
Its a long study, but this explains it a lot better then I can!
https://faculty.gordon.edu/hu/bi/te...greek/text/boyer-firstclassconditions-gtj.pdf