Question:
How is 2 Tim. 2:12 different from v. 13 where, if we are unfaithful, He abides faithful?

Answer:
2 Timothy 2:
12  If we suffer, we shall also reign with Him: if we deny Him, He also will deny us:

13  If we believe not, yet He abideth faithful: He cannot deny himself.
He can’t deny Himself—no matter how unfaithful we may be after we are saved, He can’t go back on His promise to save us by the work of Christ. Verse 13 is someone who is saved; the last part of v. 12 is someone who never was saved.