Monergism

Justification is an instantaneous legal act of God in which he (1) thinks of our sins as forgiven and Christ’s righteousness as belonging to us, and (2) declares us to be righteous in his sight.
Wayne Grudem Systematic Theology (pg. 723)

Scripture, when it treats of justification by faith, leads us in a very different direction. Turning away our view from our own works, it bids us look only to the mercy of God and the perfection of Christ. The order of justification which it sets before us is this: first, God of his mere gratuitous goodness is pleased to embrace the sinner, in whom he sees nothing that can move him to mercy but wretchedness, because he sees him altogether naked and destitute of good works. He, therefore, seeks the cause of kindness in himself, that thus he may affect the sinner by a sense of his goodness, and induce him, in distrust of his own works, to cast himself entirely upon his mercy for salvation. This is the meaning of faith by which the sinner comes into the possession of salvation, when, according to the doctrine of the Gospel, he perceives that he is reconciled by God; when, by the intercession of Christ, he obtains the pardon of his sins, and is justified; and, though renewed by the Spirit of God, considers that, instead of leaning on his own works, he must look solely to the righteousness which is treasured up for him in Christ.
John Calvin Institutes of the Christian Religion (3.11.16)

The phrase “in him” I have preferred to retain, rather than render it “by him” because it has in my opinion more expressiveness and force. For we are enriched in Christ, inasmuch as we are members of his body, and are engrafted into him: nay more, being made one with him, he makes us share with him in everything that he has received from the Father.
John Calvin Commentary on 1 Corinthians 1:5