Who are these "foolish?" I think the best answer to this question is given by our Lord himself, in the parable of the wise and foolish virgins. "The foolish" were those who had oil in their lamps, but none in their vessels. By "the foolish" in the text, therefore, we may understand those who have the light of knowledge in their heads, and the lamp of profession in their hands, but no oil of grace in their hearts. They are "foolish," because they know neither God nor themselves, neither sin nor salvation, neither the depth of the fall nor the greatness of the remedy. They are "foolish," as regards themselves, in thinking that light and knowledge will save them, without life and grace; and they are "foolish," as regards others, for want of an experimental acquaintance with the heart.
They know nothing, therefore, of the temptations of a child of God; how he is beset on every hand; how Satan is ever thrusting at or enticing him; how his own heart is continually prompting him to evil; and how snares are in every direction laid for his feet. "The foolish" know nothing of these trials; they are Pharisees, who "make clean the outside of the cup and platter," who whitewash and adorn the sepulchre without, whilst within it is "full of dead men's bones and all uncleanness."
David knew well, and every child of God knows well, that if he were allowed to slip, if he were suffered to say or do anything unbecoming, these would be the very first to make him an open reproach. "The foolish" can, and will, make no allowances for the least slip of tongue or foot, for they themselves are ignorant of the weakness of the flesh, the subtlety of Satan, the strength of sin, and the power of temptation. Were he to stumble and fall, "the foolish" would be sure to point the finger of scorn at him.
In breathing forth, then, this petition, we may well suppose him to say, "Lord, whatever temptations I may be called upon to endure, whatever snares of Satan or lusts of the flesh may beset my path behind and before, O keep me, keep me that I may not be 'the reproach of the foolish;' that they may have nothing to take hold of, to make me a bye-word, and through me to reproach thy name, cause, and truth."