“What is justification? Answer: Justification is an act of God's free grace unto sinners, in which He pardoneth all their sins, accepteth and accounteth their persons righteous in His sight; not for any thing wrought in them, or done by them, but only for the perfect obedience and full satisfaction of Christ, by God imputed to them, and received by faith alone” (Westminster Catechism, 1643).
“We thus define the Gospel justification of a sinner: It is a judicial, but gracious act of God, whereby the elect and believing sinner is absolved from the guilt of his sins, and hath a right to eternal life adjudged to him, on account of the obedience of Christ, received by faith” (H. Witsius, 1693). “A person is said to be justified when he is approved of God as free from the guilt of sin and its deserved punishment; and as having that righteousness belonging to him that entitles to the reward of life” (Jonathan Edwards, 1750).
Justification, then, refers not to any subjective change wrought in a person's disposition, but is solely an objective change in his standing in relation to the law. That to justify cannot possibly signify to make a person inherently righteous or good is most clearly to be seen from the usage of the term itself in Scripture. For example, in Proverbs 17:15 we read, “He that justifieth the wicked, and he that condemneth the just, even they both are abomination to the LORD”: now obviously he who shall make a “wicked” person just is far from being an “abomination to the LORD,” but he who knowingly pronounces a wicked person to be righteous is obnoxious to Him. Again; in Luke 7:29 we read, “And all the people that heard Him, and the publicans, justified God”: how impossible it is to make the words “justified God” signify any moral transformation in His character; but understand those words to mean that they declared Him to be righteous, and all ambiguity is removed. Once more, in 1 Timothy 3:16 we are told that the incarnate Son was “justified in (or “by”) the Spirit”: that is to say, He was publicly vindicated at His resurrection, exonerated from the blasphemous charges which the Jews had laid against Him.