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A man must do something before he can be JUSTIFIED,

Since dead men cannot regenerate themselves, you must be talking about something that occurs after regeneration. (John 3:3-8). But since anything we do is itself a gift of God, how does your "work" of doing fit in?

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"I am the vine, you are the branches. He who abides in Me, and I in him, bears much fruit; for without Me you can do nothing. (John 15:5 NKJ)

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Even our faith is not of our own "doing" but is itself a gift of God:

For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, 9 not of works, lest anyone should boast. (Ephesians 2:8-9 NKJ)

Faith is not a "work" that we "do" in order to be justified. Faith is a gift of God and it is the instrument by which God applies justification. Faith is NOT the ground of our justification nor is it something we DO to be justified. It is the means by which God justifies His elect. The GROUND or basis of justification is the objective work of Christ on the cross. The MEANS of justification is our faith, a faith that we take no credit for because it too is a divine gift given ONLY to the elect.

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Jesus answered and said to them, "This is the work of God, that you believe in Him whom He sent." (John 6:29 NKJ)

Jesus says it is the work of God that results in our believing, not our doing or working by which we give ourselves belief or faith.

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10. At length, after they have wearied themselves with perverting Scripture, they have recourse to subtleties and sophisms. One cavil is, that faith is somewhere called a work ( [John 6:29] ); hence they infer that we are in error in opposing faith to works; as if faith, regarded as obedience to the divine will, could by its own merit procure our justification, and did not rather, by embracing the mercy of God, thereby seal upon our hearts the righteousness of Christ, which is offered to us in the preaching of the gospel. Institutes of the Christian Religion, Book 3, Chapter 18, Section 10

A little below that Calvin clearly rejects faith as the "one work" that justifies a man:

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The righteousness of works consists in perfect obedience to the law. Hence you cannot be justified by works unless you follow this straight line (if I may so call it) during the whole course of your life. The moment you decline from it you have fallen into unrighteousness. Hence it appears, that righteousness is not obtained by a few works, but by an indefatigable and inflexible observance of the divine will. But the rule with regard to unrighteousness is very different. The adulterer or the thief is by one act guilty of death, because he offends against the majesty of God. The blunder of these arguers of ours lies here: they attend not to the words of James, “Whosoever shall keep the whole law, and yet offend in one point, he is guilty of all. For he that said, Do not commit adultery, said also, Do not kill,” &c. ( [James 2:10, 11] ). Therefore, it should not seem absurd when we say that death is the just recompense of every sin, because each sin merits the just indignation and vengeance of God. But you reason absurdly if you infer the converse, that one good work will reconcile a man to God notwithstanding of his meriting wrath by many sins. Institutes, Book 3, Chapter 18, section 10.

Faith is not a work we do to justify ourselves before God. It is the means God uses to apply the objective justification of the cross and Christ's active obedience to us.

Charlie

For "who has known the mind of the LORD that he may instruct Him?" But we have the mind of Christ. (1 Corinthians 2:16 NKJ)