"How does the believer know that he is justified"

Certainly not by looking at his feelings. His feelings are as changeable as the wind. Nor yet by looking at his prayers, or his good works: all that he does is mixed with sin. If he looks at himself in any way, he can find nothing that will afford a sure ground of certainty that he is justified; that is, that he is so clear of sin, that nothing can be laid to his charge for ever. Can you, my reader, with eternity before you, with the prospect of standing before that Judge who knows every secret of your life, can you say that you are clear of all sin, so clear that nothing can be laid to your charge? Are you not ready to say, "How can any sinful man in this world know that he is thus clear of all sin?" You will be astonished at the believer's simple, yet certain answer. It is this — Christ is risen.

But you will ask, "What has that to do with a believer's justification?" It has everything to do with it. "If Christ be not risen, ye are yet in your sins." (1 Cor. 15:17.) A saved sinner knows and believes the love of God in sending Jesus to be his surety and representative. His eyes have been opened to see Jesus, bearing his sins in His own body on the tree. He knows that the blood of Jesus, his surety, has met every claim of Divine holiness to the uttermost. What love and mercy to lost sinners! Now the believer can say, "As surely as Jesus was condemned for me, was delivered to death for my offences; as certainly as God dealt with Him, on the cross as my surety for my sins, so assuredly did God raise Him from the prison-house of death for my justification." Now if a surety is cast into prison for the person's debt he is bound for, when that surety comes out of prison, having paid the full demand, is not the person for whom he paid it as clear of the debt as the very person who was his surety, and paid it? And he knows he is clear of every claim. Why? Because his surety is now out of prison. Just in the same way does the believer look outside himself to Christ, his adorable surety.
Oh ponder this well: it was an awful engagement, when Jesus became the surety of all who through grace should believe on Him. Yet still He trusted God. He knew that God would justify Him from all these sins and guilt, as He says, "Thou wilt not leave my soul in hell, neither wilt thou suffer thine Holy One to see corruption." He did not leave His soul in hell; He raised Him from the dead, completely cleared from all our sins, no more to be forsaken, but to be received up to the highest glory. Now Christ had no sin to die for of His own, therefore His death was entirely for us. Just so He had no sin to be justified from of His own, therefore His resurrection also was entirely for us; He died as our surety, He rose from the dead as our representative, so that whatever God did to Christ on the cross is reckoned to the believer; and whatever God did to Christ at His resurrection, He did to us in Him as our representative. Christ is risen.

Is He perfectly and for ever clear of all sin? Even so doth God justify every believer — see Rom. 8:29-34, Heb. 10:14, 1 John 4:17. It is God that justifies.

Romans 8:29-34: For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that he might be the firstborn among many brethren. Moreover whom he did predestinate, them he also called: and whom he called, them he also justified: and whom he justified, them he also glorified. What shall we then say to these things? If God be for us, who can be against us? He that spared not his own Son, but delivered him up for us all, how shall he not with him also freely give us all things? Who shall lay any thing to the charge of God's elect? It is God that justifies. Who is he that condemns? It is Christ that died, yea rather, that is risen again, who is even at the right hand of God, who also makes intercession for us.

Hebrews 10:14: For by one offering he has perfected for ever them that are sanctified.

1 John 4:17: Herein is our love made perfect, that we may have boldness in the day of judgment: because as he is, so are we in this world.

My reader, if thou art looking at thyself in any way, thou art far from knowing that thou art justified. If the Holy Spirit shall give thee real faith in Jesus, looking entirely away from thyself at Christ, thou wilt not ask for anything to make thee more certain that thou art justified from all sin, than this one triumphant answer — Christ is risen, who is even at the right hand of God.