“Make yourselves holy.” The people were to do what it took to make themselves holy in the sight of God. This involved ritual purity, for example, doing what it took to restore physical purity in the sight of God, such as by washing or doing a necessary sacrifice, and abstaining from sex (cp. Exod. 19:15; Num. 11:18). However, more importantly, it involved “turning the heart to God, in faith and trust in His promise, and in willing obedience to His commandments, that they [Israel] should lay to heart in a proper way the miracle of grace which the Lord was about to work in the midst of them and on their behalf on the following day” (Keil & Delitzsch Commentary on the Old Testament). Thus, “making yourself holy” involved both outward and inward obedience to the commandments of God, and it is a key to God actively being in the midst of His people and working powerfully.
Jesus Christ said it this way: “Whoever has my commandments, and is keeping them, that is the one who loves me. And whoever loves me will be loved by my Father, and I will love him, and will reveal myself to him...If anyone loves me, he will keep my word, and my Father will love him, and we will come to him and make our home with him” (John 14:21, 23). God and Jesus are gracious and full of mercy, and there are many times when they move powerfully in the lives of people who have not been obedient or faithful. However, experience has proven over and over that if people want to have a consistent and powerful relationship with God and the Lord, their being purposely obedient, that is, “making themselves holy,” is a vital key to that happening.
The Schocken Bible by Everett Fox translates the phrase, “Make yourselves holy” (cp. BBE, CEB). The most common English translation, “Sanctify yourselves” is good if you understand what “sanctify” means, but to most readers the phrase is unclear.
The translation, The Scriptures, done by the Institute for Scripture Research, South Africa), has, “set yourselves apart.” While that translation captures the meaning of the word “holy” as something that is set apart, the idea of being “set apart” in this context was being set apart from the world and the unclean and ungodly things in it by obedience and dedication to God, which may not be clear to readers. Nevertheless, the translation “set yourselves apart” shows that it was something the people could do in obedience to God.
The people were to make themselves holy—purify themselves—to be able to enter into God’s presence. Yahweh is a warrior (Exod 15:3), and for Israel to fight alongside God in the battles, they must be holy.
“for tomorrow Yahweh will do wonders among you.” This is a prophecy and promise that Yahweh would do miracles among the people. That generation had seen many miracles, including manna appearing on the ground six days a week, and the pillar of cloud and fire over the Tent of Meeting (Tabernacle). Now they would see some new miracles.