William Ames: Christ is our righteousness, and faith is the instrument by which we take hold upon Christ's righteousness

William Ames explains that faith is the instrument of justification, which lays hold upon the righteousness of Christ:

Faith precedes justification as the instrumental cause, laying hold of the righteousness of Christ from which justification being apprehended follows; therefore, righteousness is said to be from faith, Rom. 9:30; 10:6. And justification is said to be by faith, Rom. 3:28. … For justifying faith goes before justification itself, as a cause goes before its effect. But faith apprehending justification necessarily presupposes and follows justification as an act follows the object towards which it is directed. … That faith is properly called justifying by which we rely upon Christ for the remission of sins and for salvation. For Christ is a sufficient object for justifying faith. Faith justifies only by apprehending the righteousness by which we are justified. That righteousness does not lie in the truth of some proposition to which we give assent, but in Christ alone Who has been made sin for us that we might be righteousness in him, 2 Cor. 5:21. … Therefore, words are often repeated in the New Testament which show that justification is to be sought in Christ alone: John 1:12; 3:15, 16; 6:40, 47; 14:1, 12; Rom. 4:5; 3:26; Acts 10:43; 26:18; and Gal. 3:26. (William Ames, Justification)