Charles Ellicott Commentary Psalms 51:4

"Against thee, thee only, have I sinned, And done that which is evil in thy sight; That thou mayest be justified when thou speakest, And be clear when thou judgest." — Psalms 51:4 (ASV)

Against you, you only ... —This can refer to nothing but a breach of the covenant relationship by the nation at large. An individual would have felt his guilt against the nation or other individuals, as well as against Jehovah. The fact that Saint Paul quotes (from the Septuagint) part of the verse in Romans 3:4 (see Note, New Testament Commentary) has naturally opened up an avenue for discussion on the bearing of the words on the doctrines of free-will and predestination. But the immediate object of his quotation appears to be to contrast the faithfulness of the God of the covenant with the falsehood of the covenant people (“Let God be true, and every man a liar”). The honor of God, as God of the covenant, was at stake. It is this thought which appears in the last clauses of this verse.

That ... — So that (or, in order that) you are (or may be) justified in your cause, and clear in your judgment. The Hebrew, rendered in the Authorized Version when you speak, is often used of a cause or suit (see Exodus 18:16-22, “matter,” etc.), and it is here plainly used in this sense and is parallel to judgment. The clause seems to imply not only a sense of a breach of the covenant, but some manifest judgment from Jehovah in consequence; and, as usual, it is of its effect on the nations that the psalmist thinks. The Divine honor would be justified when the suffering nation confessed that condemnation and punishment had been deserved. This was apparently the meaning read in the words by the Septuagint.