On this occasion Jesus undoubtedly had in mind the law of God at Leviticus 6:2-7, which reads:

“In case a soul sins in that he does behave unfaithfully toward Jehovah and does deceive his associate about something in his charge or a deposit in hand or a robbery or he does defraud his associate,

In these cases mentioned in the Law, an Israelite had defrauded his associate in some way, quite seriously. He knew that his associate had something against him. It was not an imagined wrong, but a real one. Now, if the wrongdoer went to the temple to offer a sacrifice, claiming to perform worship and service to Jehovah, of what value would it be if he went before Jehovah as a thief, a liar or an extortioner? God would not view his offering with favor and bestow a blessing; as he later told King Saul through the prophet Samuel: “Does Jehovah have as much delight in burnt offerings and sacrifices as in obeying the voice of Jehovah? Look! To obey is better than a sacrifice, to pay attention than the fat of rams.”​—1 Sam. 15:22.

Jesus roundly condemned the scribes and Pharisees on the same grounds, saying: “Woe to you, scribes and Pharisees, hypocrites! because you give the tenth of the mint and the dill and the cummin, but you have disregarded the weightier matters of the Law, namely, justice and mercy and faithfulness.”​—Matt. 23:23.

For, as the law pointed out, such a law violator had behaved unfaithfully, first, “toward Jehovah.”​—Lev. 6:2.