If we deny him, he will also deny us A threatening is likewise added, for the purpose of shaking off sloth; for he threatens that they who, through the dread of persecution, leave off the confession of his name, have no part or lot with Christ. How unreasonable is it, that we should esteem more highly the transitory life of this world than the holy and sacred name of the Son of God! And why should he reckon among his people those who treacherously reject him? Here the excuse of weakness is of no value; [1] for, if men did not willingly deceive themselves with vain flatteries, they would constantly resist, being endued with the spirit of strength and courage. Their base denial of Christ proceeds not only from weakness, but from unbelief; because it is in consequence of being blinded by the allurement of the world, that they do not at all perceive the life which is in the kingdom of God. But this doctrine has more need of being meditated on than of being explained; for the words of Christ are perfectly clear, "Whoever shall deny me, him will I also deny" (Matthew 10:33.) It remains that every one consider with himself, that this is no childish terror, but the judge seriously pronounces what will be found, at the appointed time, to be true.

Footnotes

1 - "On ne gaigne rien yci de se defendre et excuser, en alleguant son infirmite." -- "Here nothing is gained by defending and excusing ourselves on the ground of our weakness."

If we suffer, we shall also reign with him - The meaning is, that the members will be treated as the Head is. We become united with him by faith, and, if we share his treatment on earth, we shall share his triumphs in heaven; see the notes at Romans 8:17.
If we deny him, he also will deny us; - see the notes at Matthew 10:32-33.

If we suffer - with him - These are other parts of the true doctrine, which the apostle mentions above.

If we suffer,.... With him, with Christ, as in Romans 8:17 all the elect suffered with Christ when he suffered; they suffered in him the whole penalty of the law, all the righteousness, strictness, and severity of it; and they are partakers of the benefits of his sufferings, as peace, pardon, righteousness, redemption, and everlasting salvation. And such being called by grace, and having made a profession of Christ, they suffer shame and reproach, loss of credit and reputation, and sometimes loss of goods, and corporeal punishment, and even death itself: but though they do, and if they should, they may be satisfied of the truth of this,
we shall also reign with him; they reign with him now in the kingdom of grace; grace reigns in their hearts, where Christ, the King of glory, has entered, and has set up his throne, and where he dwells by faith, they being made kings and priests unto God by him; and they shall reign with him in his kingdom here on earth, for the space of a thousand years; and they shall reign with him in glory to all eternity: this is certain, for this kingdom is prepared for them, it is given to them, they are called unto it, and have both a right unto, and meetness for it; see Romans 8:17,
if we deny him, he also will deny us: there is a denying of Christ in words; so it is denied by the Jews that Christ is come in the flesh, and that Jesus is the Messiah; and some that have bore the Christian name, though very unworthily, have denied his true deity, his real humanity, proper sonship, and the efficacy of his blood, righteousness, and sacrifice, for pardon, justification, and atonement: and there is a denying of him in works; so some that profess to know him, and do own him in his person and offices, yet in works deny him; their conversation is not becoming their profession of him; they have the form of godliness, but deny the power of it: there is a secret and silent denying of him, when men are ashamed of him, and do not confess him; and there is an open denying of him, by such who set their mouth against the heavens, and their tongue walketh throughout the earth; there is a partial denying of Christ, which was Peter's case, though his faith in him, and love to him, were not lost; and there is a total denying of him, a thorough apostasy, and from which there is no recovery; and if there be any such apostates among those who have named the name of Christ, he will deny them, he will not own them for his another day; he will set them at his left hand; he will declare he knows them not, and will banish them from his presence for evermore. This is another branch of the faithful saying; this will certainly be the case; Christ himself has said it, Matthew 10:33.

suffer--rather, as the Greek is the same as in 2-Timothy 2:10, "If we endure (with Him)" (Romans 8:17).
reign with him--The peculiar privilege of the elect Church now suffering with Christ, then to reign with Him (see on 1-Corinthians 6:2). Reigning is something more than mere salvation (Romans 5:17; Revelation 3:21; Revelation 5:10; Revelation 20:4-5).
deny--with the mouth. As "believe" with the heart follows, 2-Timothy 2:12. Compare the opposite, "confess with thy mouth" and "believe in thine heart" (Romans 10:9-10).
he also will deny us-- (Matthew 10:33).

If we deny him - To escape suffering for him.