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 Romans 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 Romans 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.

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, then (o) thou consentedst with him, and hast been partaker with adulterers.
(o) He shows what the fruits of them who contemn God's word are.

When thou sawest a thief, then thou consentedst with him,.... Or "didst run with him" (a); joined and agreed with him in the commission of the same things; which was literally true of the Scribes and Pharisees: they devoured widows' houses, and robbed them of their substance, under a pretence of long prayers; they consented to the deeds of Barabbas, a robber, when they preferred him to Jesus Christ; and they joined with the thieves on the cross in reviling him: and, in a spiritual sense, they stole away the word of the Lord, every man from his neighbour; took away the key of knowledge from the people, and put false glosses upon the sacred writings;
and hast been a partaker with adulterers; these teachers of the law were guilty both of theft and adultery, Romans 2:21; they are called by our Lord an adulterous generation, Matthew 12:39; and they were so in a literal sense; see John 8:4; and in a figurative one, adulterating the word of God, and handling it deceitfully.
(a) , Sept. "currebas cum eo", V. L. sic Eth. Syr. Targum, so Vatablus, Musculus, Piscator, Ainsworth.