Behold, thou desirest truth in the inward parts - The word rendered “desirest,” means to have pleasure in; to delight in; and the idea is that this only is agreeable to God, or this only accords with his own nature. The word rendered “inward parts,” means properly the reins, and is usually employed to denote the seat of the mind, the feelings, the intellect. Compare the notes at Job 38:36. The allusion is to the “soul;” and the idea is, that God could be satisfied with nothing “but” purity in the soul. The “connection” is this: David was deeply conscious of his own pollution; his deep, early, native depravity. This, in his own mind, he contrasted strongly with the nature of God, and with what God must require, and be pleased with. He “felt” that God could not approve of or love such a heart as his, so vile, so polluted, so corrupt; and he felt that it was necessary that he should have a pure heart in order to meet with the favor of a God so holy. But how was that to be obtained? His mind at once adverted to the fact that it could come only from God; and hence, the psalm now turns from confession to prayer. The psalmist pleads earnestly Psalms 51:7-10 that God “would” thus cleanse and purify his soul.
And in the hidden part - In the secret part; the heart; the depths of the soul. The cleansing was to begin in that which was hidden from the eye of man; in the soul itself. Wisdom, heavenly, saving wisdom, was to have its seat there; the cleansing needed was not any mere outward purification, it was the purification of the soul itself.
Thou shalt make me to know wisdom - Thou only canst enable me to understand what is truly wise. This wisdom, this cleansing, this knowledge of the way in which a guilty man can be restored to favor, can be imparted only by thee; and “thou wilt do it.” There is here, therefore, at the same time a recognition of the truth that this “must” come from God, and an act of faith, or a strong assurance that he “would” impart this.