What is the efficient cause of our justification?
The whole Trinity. All the persons in the blessed Trinity have a hand in the justification of a sinner: opera Trinitatis ad extra sunt indivisa. God the Father is said to justify. 'It is God that justifieth.' Rom viii 33. God the Son is said to justify. 'By him all that believe are justified.' Acts xiii 39. God the Holy Ghost is said to justify. 'But ye are justified by the Spirit of our God.' I Cor vi ii. God the Father justifies, as he pronounces us righteous; God the Son justifies, as he imputes his righteousness to us; and God the Holy Ghost justifies, as he clears up our justification, and seals us up to the day of redemption.
-Thomas Watson
Appropriation, or applying Christ to ourselves. A medicine, though it be ever so sovereign, if not applied, will do no good. Though the plaster is made of Christ's own blood, it will not heal, unless applied by faith; the blood of God, without faith in God, will not save. This applying of Christ is called receiving him. John 1:12. The hand receiving gold, enriches. Just so, the hand of faith, receiving Christ's golden merits with salvation, enriches us.
-Thomas Watson