In the context of conversion (initial justification), who or what exactly makes and declares us to be righteous in God’s sight? In other words, what is the primary (efficient) cause of our justification? Is it faith? Is it God? Is it both?
First, it is entirely necessary to be properly disposed in order for God to justify an ungodly sinner. Acquired faith, rather than infused faith, is a disposition and the instrument by which we receive the unmerited gift of justification. However, it is God who justifies us, making and declaring us righteous in His sight. Strictly speaking, faith does not justify us, God does. In other words, mere acquired faith does not convert us or bring us from a state of spiritual death into supernatural life, faith being only a disposition and instrument necessary for initial justification. This is why Saint Paul incessantly says that we are justified BY and/or THROUGH faith, as opposed to saying that faith justifies us:
“For we hold that one is justified by faith apart from works of the law.” – Romans 3:28 ESV
“[…] yet we know that a person is not justified by works of the law but through faith in Jesus Christ.” – Galatians 2:16 ESV
Moreover, Saint Paul clearly states that it is God who justifies us:
“[S]o as to be just and the one [God] who justifies those who have faith in Jesus.” – Romans 3:26 ESV
“And to the one who does not work but believes in him who justifies the ungodly.” Romans 4:5 ESV
Notice, God can only justify those with the proper disposition of faith, i.e. “those who have faith in Jesus” and, “but believes”, respectively; an unbeliever mockingly asking God to justify him/her, or simply anyone in vain attempting to put God to the test would make it impossible for God to justify that person. God’s justifying grace is inherently sufficient, however, what makes it efficacious to the soul is the required disposition of faith.
A perfect correlating parallel can be seen in the public life of Jesus’ miracles of healing and forgiving. Notice, in almost every instance of Jesus healing and/or forgiving, the required disposition of faith was demanded by Christ; it wasn’t only the interior disposition of faith that was required, but an external manifestation of that faith through an act of faith. This is why even the disposition of our cooperation of the will is necessary for God justifying us.
For example, when Jesus healed individuals of their infirmities or forgave their sins, He declared, “Your faith has saved you”; what He means is that it was through the instrument of faith that they received what Christ offered and ultimately performed. We could ask: “Did Jesus save or was it their faith that saved?” In a sense it was both, however it was Jesus who primarily saved (healed and forgave). Jesus can only heal those with the required and necessary disposition of faith, hence why He asks, “Do you believe I can do this?” to the blind beggars in Matthew 9:28, and He confirmed this necessary disposition by continuing, “According to your faith be it done to you.” Jesus conferred the healing remedy of sight to the blind men, but it was received through the disposition of their faith.
It is important to note that the required and necessary disposition of faith “does not itself constitute the grace of justification, nor does it establish any claim to it.”1 In the context of initial justification, or conversion from spiritual death to life, the instrumental means is the disposition of faith that leads to God justifying in through the Sacrament of Holy Baptism; God has bound Himself to justify the sinner through these means, but again, it is God who justifies as the efficient and primary cause. God justifies us, and it is through and by the disposition of faith that receives and makes this act of God efficacious to the individual person.
1 THE MOST REV. JOHN MACEVILLY D.D, An Exposition of the Epistles of Saint Paul and of the Catholic Epistles: Volumes 1 & 2