It is a faithful saying - Or, rather, that which he was about to say was worthy of entire credence and profound attention; see the notes at 1 Timothy 1:15. The object is to encourage Timothy to bear trials by the hope of salvation.
For if we be dead with him - see the notes at Romans 6:8.
We shall also live with him - This was a sort of maxim, or a settled point, which is often referred to in the Bible; see the Romans 6:3-5 notes; John 11:25 note; 1 Thessalonians 4:14 note.