II. However, we must premise here that God, the just Judge (dikaiokritēn), cannot pronounce anyone just and give him a right to life except on the ground of some perfect righteousness which has a necessary connection with life; but that righteousness is not of one kind. For as there are two covenants which God willed to make with men—the one legal and the other of grace—so also there is a twofold righteousness—legal and evangelical. Accordingly there is also a double justification or a double method of standing before God in judgment—legal and evangelical. The former consists in one’s own obedience or a perfect conformity with the law, which is in him who is to be justified; the latter in another’s obedience or a perfect observance of the law, which is rendered by a surety in the place of him who is to be justified—the former in us, the latter in Christ. Concerning the first, Paul says, “Not the hearers, but the doers of the law shall be justified” (Rom. 2:13); and “Moses describeth the righteousness which is of the law. That the man which doeth those things shall live by them” (Rom. 10:5). Concerning the other, he says, “The gospel is the power of God unto salvation to every one that believeth, for therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith” (Rom. 1:16, 17); and “Being justified freely by his grace through the redemption that is in Christ Jesus” (Rom. 3:24). Concerning both, he says, “That I may be found in Christ, not having my own righteousness, which is of the law, but that which is through the faith of Christ” (Phil. 3:9; cf. also Rom. 9:30, 31). Hence a twofold justification flows: one in the legal covenant by one’s own righteousness according to the clause, “Do this and live”; the other in the covenant of grace, by another’s righteousness (Christ’s) imputed to us and apprehended by faith according to the clause, “Believe and thou shalt be saved.” Each demands a perfect righteousness. The former requires it in the man to be justified, but the latter admits the vicarious righteousness of a surety. The former could have place in a state of innocence, if Adam had remained in innocence. But because after sin it became impossible to man, we must fly to the other (i.e, the gospel), which is founded upon the righteousness of Christ.
Francis Turretin, Institutes of Elenctic Theology, ed. James T. Dennison Jr., trans. George Musgrave Giger (Phillipsburg, NJ: P&R Publishing, 1992–97), 16.2.2. (2.637).