Q: What is justification?
A: Justification is an act of God's free grace, wherein he pardoneth all our sins, and accepteth us as righteous in his sight, only for the righteousness of Christ imputed to us, and received by faith alone.

Ephesians 1:7. In whom we have redemption through his blood, the forgiveness of sins, according to the riches of his grace.
2 Corinthians 5:19, 21. To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation. . . For he hath made him to besin for us, who knew no sin; that we might be made the righteousness of God in him.
Romans 4:5. But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness.
Romans 3:22-25. Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference: for all have sinned, and come short of the glory of God; being justified freely by his grace through the redemption that is in Christ Jesus: whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God.
Romans 5:17-19. For if by one man's offence death reigned by one, much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.) therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life. For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous.
Romans 4:6-8. Even as David also describeth the blessedness of the man, unto whom God imputeth righteousness without works, saying, Blessed are they whose iniquities are forgiven, and whose sins are covered. Blessed is the man to whom the Lord will not impute sin.
Romans 5:1. Therefore being justified by faith, we have peace with God through our Lord Jesus Christ.
Acts 10:43. To him give all the prophets witness, that through his name whosoever believeth in him shall receive remission of sins.
Galatians 2:16. Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified.
Philippians 3:9. And be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith.

Westminster Project - See Francis Beattie on Justification as a benefit of Redemption