Psalms 50:18

They are unworthy to use even the words of God's law. Their hypocrisy and vice are exposed by illustrations from sins against the seventh, eighth, and ninth commandments. God, no longer (even in appearance) disregarding such, exposes their sins and threatens a terrible punishment.

When thou sawest a thief, then thou consentedst with him. Moral honesty cannot be absent where true grace is present. Those who excuse others in trickery are guilty themselves; those who use others to do unjust actions for them are doubly so. If a man be ever so religious, if his own actions do not rebuke dishonesty, he is an accomplice with thieves. If we can acquiesce in anything which is not upright, we are not upright ourselves, and our religion is a lie. And hast been partaker with adulterers. One by one the moral precepts are thus broken by the sinners in Zion. Under the cloak of piety, unclean livers conceal themselves. We may do this by smiling at unchaste jests, listening to indelicate expressions, and conniving at licentious behaviour in our presence; and if we thus act, how dare we preach, or lead public prayer, or wear the Christian name? See how the Lord lays righteousness to the plummet. How plainly all this declares that without holiness no man shall see the Lord! No amount of ceremonial or theological accuracy can cover dishonesty and fornication: these filthy things must be either purged from us by the blood of Jesus, or they will kindle a fire in God's anger which will burn even to the lowest hell.

Here the Lord turns to the manifestly wicked among his people; and such there were even in the highest places of his sanctuary. If moral formalists had been rebuked, how much more these immoral pretenders to fellowship with heaven? If the lack of heart spoiled the worship of the more decent and virtuous, how much more would violations of the law, committed with a high hand, corrupt the sacrifices of the wicked?

When thou sawest a thief - Rapine, adulteries, and adulterous divines, were common among the Jews in our Lord’s time. The Gospels give full proof of this.

When thou sawest a thief - When you have seen or found one who was intending to commit theft, then (instead of rebuking or exposing him) you have been willing to act with him, and to divide the profits. The words “when thou sawest” would seem to imply readiness and willingness to engage with them, as “at first sight.” Whenever there was an opportunity to share in the results of theft, they were ready to engage in it. The main “point” in this is, that they were willing to do so even when observing the outward duties of religion, and when professing to be the true worshippers of God. A similar sentiment occurs in Rom 2:21. See the notes at that passage. Then thou consentedst with him - literally, Thou didst delight in him, or hadst pleasure in him. He was a man after thine own heart. Thou wast at once on good terms with him. And hast been partaker with adulterers - Margin, as in Hebrew, “thy portion was with adulterers.” This was a common vice among the Jewish people. See the notes at Rom 2:22. The idea here is, that they were associated in practice with adulterers; they were guilty of that crime as others were. The point of the remark here is, that they did this under the cloak of piety, and when they were scrupulous and faithful in offering sacrifices, and in performing all the external rites of religion.