“ Get up, sanctify the people and say, ‘Sanctify yourselves against tomorrow, for thus says YHWH, the God of Israel, there is a devoted thing in your midst, Oh Israel. You cannot stand before your enemies, until you take away the devoted thing from among you.' ”
So Joshua was commanded to rise and do something about it. YHWH would assist in the search for the devoted thing which was such a curse to them, but they must first sanctify themselves to prepare for His drawing near. This probably meant washing their clothes, bathing with water, waiting in their tents until the evening and abstention from sexual relations and from anything unclean. They were also to be made aware of the seriousness of the situation. It may well also have included special sacrifices and offerings on their behalf at the Tabernacle.
“YHWH, the God of Israel.” This phrase occurs previously only in Exodus 5:1; Exodus 32:27. It was used at particularly solemn moments. In Exodus 5:1 it was at the time of Moses' very first demand to Pharaoh in YHWH's name. In Exodus 32:27 it was used in the giving of the command to the Levites to slay Israelites caught in idolatry when Moses came down from Sinai. It became prominent in the book of Joshua, in the historical books and especially in Jeremiah.