V. The covenant of nature is that which God the Creator made with innocent man as his creature, concerning the giving of eternal happiness and life under the conditions of perfect and personal obedience. It is called “natural,” not from natural obligation (which God does not have towards man it is also called “legal” because the condition on man’s part was the observation of the law of nature engraved within him; and of works because it depended upon works or his proper obedience.
XIV. Although man was already bound to this obedience by a natural obligation as a rational creature, necessarily subject to the dominion of God and his law, yet he was more strongly bound by a federal obligation which God so stipulated that man–by the powers received in creation–could perform it, although in order that he might actually perform it, he still needed the help of God both to actuate these faculties and powers and to preserve them from change. This help did not tend to the infusion of any new power but only to exercising the efficacy of that power which he had received. Now this did not belong properly to the covenant of nature, but always depended on the most free good pleasure (eudokia) of God; Otherwise the covenant of nature has been immutable, And man had never sinned.
XV. The sanction of the covenant attended the exaction of duty. It consisted both in the promise of reward and gain and in the threatening of punishment. The promise was of the highest happiness (of eternal life) to be passed not on earth but in heaven. The threatening was of death and whatever in scripture comes under the name of death to express punishment of all kinds (into which man by his own sin deservedly fell).
XVI. However, from this pact arises the mutual obligation of the parties, differing according to their condition. With respect to man, not only was it from the pact, but absolute and simple from the nature of the thing (and on account of God, to whom man as a creature to the Creator, the beneficiary to the Benefactor, owed himself wholly and whatever he had to God and was bound to love him with his whole heart). But with respect to God, it was gratuitous, as depending upon a pact or gratuitous promise (by which God was bound not to man, but to himself and to his own goodness, fidelity and truth, Rom. 3:3; 2 Tim 2:13). Therefore there was no debt (properly so called) from which man could derive a right, but only a debt of fidelity, arising out of the promise by which God demonstrated his infallible and immutable constancy in truth. If the apostle seems to acknowledge this right or debt (Rom. 4:4), it must be understood in no other than a respective sense; not as to the proportion and condignity of the duty rendered to God by man (Rom. 8:18; Lk. 17:10), but to the pact of God and justice (i.e., the fidelity of him making it).
XVII. If therefore upright man in that state had obtained this merit, it must not be understood properly and rigorously. Since man has all things from and owes all to God, he can seek from him nothing as his own by right, nor can God be a debtor to him–not by condignity of work and from its intrinsic value (because what ever that may be, it can bear no proportion to the infinite reward of life), but from the pact and the liberal promise of God (according to which man had the right of demanding the reward to which God had of his own accord bound himself) and in comparison with the covenant of grace (which rests upon the sole merit of Christ, by which he acquired for us the right to life). However, this demanded antecedently a proper and personal obedience by which he obtained both his own justification before God and life, as the stipulated reward of his labors.
—Francis Turretin, Institutes of Elenctic Theology, trans. George Musgrave Giger (Phillipsburg: P&R Publishing, 1992), Eighth topic, Q. 3, §§5, 14-17 . (vol. 1.575, 577–78).