Petrus Van Mastricht on Good Works

Petrus Van Mastricht gives a scriptural basis on how to evaluate whether particular actions are good works or not.

For their constitution these three things are conjointly required:

1. that they flow forth from a regenrate or renewed heart (1 Tim. 1:5; Matt. 12:35), because a bad tree cannot bear good fruits (Matt. 7:18), and that which is born of the flesh is nothing but flesh (John 3:6), just as from a depraved heart proceeds nothing but depraved thoughts (Matt. 15:19); and not only from a regenerate heart, but also from a believing heart, or from faith unfeigned (1 Tim. 1:5), because without faith it is impossible to please God (Heb. 11:6); indeed, that which does not result from faith, however attractive it may be, at least before God is sin (Rom. 14:23), and our best works, apart from Christ apprehended by faith, in the divine sight are nothing but menstrual rags (Isa. 64:6). For unless that imperfection which adheres to our best works is mended by Christ's pefection, and that impurity which adheres to them from our natural impurity is covered by the purity of Christ's righteounsess, they are an abomination to God in which sense it is said that apart from Christ we can do nothing, namely, that is pleasing to God (John 15:5); but we are in Christ, and, are united with him, bu faith (Eph. 3:17).
2. That they be directed by us to the glory of God as the ultimate end(1 Cor. 10:31; Matt. 5:16; John 15:8; Phil. 1:11), by lack of which works of the Pharisees and other hypocrites, however otherise atttracteve, are rejected by Christ (Matt. 6:2, 5; 23:5). Not tha it is precisely necessary that this glory of God always be actuallly intended (although this also, as much as can be ought to be done carefully by those perfomring the works), but that the virtual and general ultimate goal of all our works ought to be the glorification of God (1 Cor. 6:20).
3. That they be supplied according to the prescription of the divine law (Gal. 6:16, ; Isa. 8:20; Deut 4:2; Isa. 29:13; Matt. 15:9; Col. 2:22). For if they are not prescribed by him they do not render an obedience pleasing to God; for which reason God says, "Who has required this from your hand? (Isa. 1:12).

(Petrus Van Mastricht, Theoretical-Practical Theology: The Application of Redemption and the Church, Volume 5, pp. 259,260)