Here’s Johannes Wollebius,
“VI. The instrumental cause is the word of the gospel. ‘It is the power of God for salvation to everyone that believes’ (Rom. 1:16).
VII. For justification, considered in its passive aspect, with respect to the person who is justified, there is no cause except the instrumental one, faith.
VIII. The expression ‘We are justified by faith’ is a metonymy, and has the same meaning as ‘We are justified by the merit of Christ, which is apprehended by faith.’
IX. Faith is said to justify ‘alone’ [sola] by contrast with works, which, although they are effects resulting from faith, are not a cause of justification, and do not precede justification, but follow it.
Although this specific word ‘alone’ does not occur in Scripture, it is expressed in synonymous phrases. Such phrases are ‘without works’ (Rom. 3:28), ‘by grace as a gift’ (Rom. 3:24), ‘only by faith’ (Eph. 2:8, Gal. 2:16). Therefore, although faith does not exist by itself, but is united to works, nevertheless faith alone justifies, just as the sun does not exist alone in the heavens, but it alone makes day.
X. Faith justifies, not as a work or because of its worthiness, but simply as the instrument that apprehends Christ.” (165, 166 Compendium Theologiae Christianae)