And the LORD said unto Moses, Go unto the people, and {e} sanctify them to day and to morrow, and let them wash their clothes,

(e) Teach them to be pure in heart, as they show themselves outwardly clean by washing.

Wesley's Notes

19:10 Sanctify the people - As Job before sent and sanctified his sons, Job 1:5. Sanctify them, that is, call them off from their worldly business, and call them to religious exercises, meditation and prayer, that they may receive the law from God's mouth with reverence and devotion. Two things particularly were prescribed as instances of their preparation. 1st, In token of cleansing of themselves from all sinful pollutions, they must wash their clothes. Not that God regards our clothes, but while they were washing their clothes, he would have them think of washing their souls by repentance. It becomes us to appear in clean clothes when we wait upon great men; so clean hearts are required in our attendance on the great God. 2dly, In token of their devoting themselves entirely to religious exercises upon this occasion they must abstain even from lawful enjoyments during these three days, and not come at their wives.

Matthew Henry's Concise Commentary

19:9-15 The solemn manner in which the law was delivered, was to impress the people with a right sense of the Divine majesty. Also to convince them of their own guilt, and to show that they could not stand in judgment before God by their own obedience. In the law, the sinner discovers what he ought to be, what he is, and what he wants. There he learns the nature, necessity, and glory of redemption, and of being made holy. Having been taught to flee to Christ, and to love him, the law is the rule of his obedience and faith.