If this principle be correct, if the law demands entire conformity to the nature and will of God, it follows:
1. That there can be no perfection in this life. Every form of perfectionism which has ever prevailed in the Church is founded either on the assumption that the law does not demand entire freedom from moral evil, or upon the denial that anything is of the nature of sin, but acts of the will. But if the law is so extensive in its demands as to pronounce all defect in any duty, all coming short in the purity, ardor, or constancy of holy affections, sinful, then there is an end to the presumption that any mere man since the fall has ever attained perfection.
2. It follows also from this principle that there can never be any merit of good works attributable to men in this world. By merit, according to the Scriptural sense of that word, is meant the claim upon reward as a matter of justice, founded on the complete satisfaction of the demands of the law. But if those demands never have been perfectly fulfilled by any fallen man, no such man can either be justified for his works, or have, as the Apostle expresses it, any καύχημα [kauchema, boasting], any claim founded on merit in the sight of God. He must always depend on mercy and expect eternal life as a free gift of God.
3. Still more obviously does it follow from the principle in question that there can be no such thing as works of supererogation. If no man in this life can perfectly keep the commandments of God, it is very plain that no man can do more than the law demands. The Romanists regard the law as a series of specific enactments. Besides these commands which bind all men there are certain things which they call precepts, which are not thus universally binding, such as celibacy, poverty, and monastic obedience, and the like. These go beyond the law. By adding to the fulfillment of the commands of God, the observance of these precepts, a man may do more than is required of him, and thus acquire an amount of merit greater than he needs for himself, and which in virtue of the communion of saints, belongs to the Church, and may be dispensed, through the power of the keys, for the benefit of others. The whole foundation of this theory is of course removed, if the law demands absolute perfection, to which, even according to their doctrine, no man ever attains in this life. He always is burdened with venial sins, which God in mercy does not impute as real sins, but which nevertheless are imperfections.
Charles Hodge, Systematic Theology, volume 2, pages 185-86 (1872 ed.)
3. It is in this doctrine that the most essential difference lies between the covenant of grace and the first covenant. The adverse scheme of justification supposes that we are justified by our works, in the very same sense wherein man was to have been justified by his works under the first covenant. By that covenant our first parents were not to have had eternal life given them for any proper merit in their obedience; because their perfect obedience was a debt that they owed God. Nor was it to be bestowed for any proportion between the dignity of their obedience, and the value of the reward; but only it was to be bestowed from a regard to a moral fitness in the virtue of their obedience, to the reward of God's favor; and a title to eternal life was to be given them, as a testimony of God's pleasedness with their works, or his regard to the inherent beauty of their virtue. And so it is the very same way that those in the adverse scheme suppose that we are received into God's special favor now, and to those saving benefits that are the testimonies of it. I am sensible the divines of that side entirely disclaim the Popish doctrine of merit; and are free to speak of our utter unworthiness, and the great imperfection of all our services. But after all, it is our virtue, imperfect as it is, that recommends men to God, by which good men come to have a saving interest in Christ, and God's favor, rather than others; and these things are bestowed in testimony of God's respect to their goodness. So that whether they will allow the term merit or no, yet they hold, that we are accepted by our own merit, in the same sense, though not in the same degree, as under the first covenant.
(Works of President [Jonathan] Edwards, Volume 5 (of 10), pp. 447-48, 1829 ed.)