Romans 4:5

But to him that worketh not
Not that the believer does not work at all, but not from such principles, and with such views as the other; he does not work in order to obtain life and salvation; he does not seek for justification by his doings:

but believeth on him that justifieth the ungodly:
or that ungodly one: particular reference is had to Abraham, who in his state of unregeneracy was an ungodly person; as all God's elect are in a state of nature, and are such when God justifies them, being without a righteousness of their own; wherefore he imputes the righteousness of another, even that of his own Son, unto them: and though he justifies the ungodly, he does not justify their ungodliness, but them from it; nor will he, nor does he leave them to live and die in it; now to him that worketh not, that is perfect righteousness; or has no opportunity of working at all; or what he does, he does not do, that he might be justified by it; but exercises faith on God as justifying persons, who, like himself, are sinners, ungodly and destitute of a righteousness:

his faith is counted for righteousness;
not the act, but the object of it; which was Abraham's case, and therefore was not justified by works. The Vulgate Latin version here adds, "according to the purpose of the grace of God".