we shall … live with Him (v.11) = lit. “we shall live by means of Him” (Romans 6:8)
endure (v.12) = persevere
reign (v.12) = reign as king
deny (v.12) = disown
I believe Paul is using a play on words here. If we deny (disown) Him, He also will deny (deny us the reward we no longer deserve) us … He cannot deny (be unfaithful to) Himself.
deny (v.13) = prove false to oneself
“For if we be dead with Him, we shall also live with Him.” i.e., if by faith we recognize His death as our death, and have thus died with Him (Romans 6:3), we shall also share in His resurrection life and thus in His “eternal glory.” All this is part of the special message of grace committed to the apostle Paul.
But in addition to salvation by grace, there are rewards the believer may earn by faithful conduct and service for Christ. Thus the apostle goes on to say: “If we suffer[i.e., with Him], we shall also reign with Him; if we deny Him, He also will deny us (v.12).
This has nothing to do with salvation by grace, but rather with our Christian conduct and testimony, by which we may one day “receive a reward” or “suffer loss” (1 Corinthians 3:14-15). As if anticipating any doubts which believers may entertain as to this, Paul continues:If we believe not, yet He abideth faithful: He cannot deny Himself (v.13).
The words “believe not” here are the opposite of the word “faithful” in the same verse. Indeed, the words “belief” and “faith” are often used interchangeably in the KJV. Thus the sense is: “If we are unfaithful, yet He abideth faithful: He cannot deny Himself.” And thus salvation by grace, through faith, is distinguished from rewards for faithfulness.” — Stam, page 184.
The context of this passage is Paul encouraging Timothy to