VIII. New Obedience; Good Works
New obedience is exercised through good works. Moreover, there are good works, voluntary actions of the sanctified, which come from a pure heart, love of God, true faith in Christ, according to the law of God, for His glory, one’s own salvation, and the usefulness of neighbor. For in order that a work be good, concursus of all causes is required. This includes its beginning which is the disposition of the will or the “treasure of a good heart” (Matt. 12:35), “a good tree” (Matt. 12:7, 17), a “pure heart” (1 Tim. 1:5), conformity with the law of God because those things “must be done” which “God commands” (Deut. 12:32), and also rectitude of circumstances because goods are necessary to be conducted well. Moreover, love of God is introduced because “love is the fulfillment of the law” (Rom. 13:10) and the root of good works (1 Cor. 13:4). Faith is also introduced because “whatever is not from faith is sin” (Rom. 14:23), and “love from faith is not feigned” (1 Tim. 1:5). The end is intention of right which the person who wills orders to the glory of God and salvation of neighbor, according to His Word. “If your eye is simple, the whole body will be lucid” (Matt. 6:22). But the good works noted in the latter are distinguished from the works of the unregenerate, Gentiles, and hypocrites. Moreover, the works exacted even of the regenerate for the rigor of the same bring forth their imperfection in this life, so that none can be found without adhering vice. (XXII–XXX).

IX. The Necessity of Those
Good works of adults and of those who can produce them are in a sense as necessary as sanctification itself. For although they do not justify, they are nevertheless fruits of justifying faith. They do not indeed serve the interests of meriting salvation, but taking possession of it. They are a consequence of the adjoined antecedent, faith. “Let your light so shine before men, that they may see your good works, and glorify your Father, who is in heaven” (Matt. 5:16). (XXX).

X. They Do Not Effect or Merit Salvation
Yet good works neither effect nor merit eternal life. For “eternal life” is “a gift of God” (Rom. 6:23), opposed “to debt” (ὀφειλήματι) (Rom. 4:4). Moreover, it depends on Christ alone and faith in Him (John 17:3, Acts 4:12), and it is given as “an inheritance” (Matt. 25:34, Rom. 8:17, Eph. 1:18). For all boasting must be excluded from salvation. “Where is boasting? It is excluded through the law of faith” (Rom. 3:27). But the one who merits can boast. Yet it is “not from works, lest anyone boast” (Eph. 2:9). Finally, works merit neither from condignity, nor from pact, nor from congruency. They do not merit from condignity because not even “our sufferings,” still less actions, “are worthy of the glory of God to be revealed in us” (Rom. 8:18), and we are debtors of doing those things for God by right of creation and redemption. God has no profit when “we render our ways perfect” (Job 22:2, 3), and “we are the workmanship of God, created in Christ Jesus for good works” (Eph. 2:10). They do not merit from pact because God nowhere promised life to one meriting it, and legal sin cannot be entered from pact. They do not merit from congruity because God promised the reward of merit not apart from perfect obedience, as from merit. (XXXI–XXXVII).

J. H. Heidegger, The Concise Marrow of Theology (1697), vol. 4. Classic Reformed Theology, trans. Casey Carmichael (Grand Rapids: Reformation Heritage Books, 2019), 163–64.