Better, faithful is the saying. See on 1Ti 1:15. It refers to what precedes - the eternal glory of those who are raised with Christ (2Ti 2:8) which stimulates to endurance of sufferings for the gospel.
For (γὰρ)
Faithful is the saying that the elect shall obtain salvation with eternal glory, for if we be dead, etc. The following words to the end of 2Ti 2:12 may be a fragment of a hymn or confession, founded on Rom 6:8; Rom 8:17.
If we be dead with him (εἰ συναπεθάνομεν)
A.V. misses the force of the aorist. Better, if we died, etc. Comp. Rom 6:8; Col 2:20. For the verb, comp. Mar 14:31; 2Co 7:3.