What does it mean that believers are set apart for God's use?
Believers are holy, set apart exclusively for God's purposes, just as priestly utensils were not for common use. They are set aside and intrinsically in His program to be used as He sees fit because they are His, holy and without blame before Him even while still having the old sin nature.
Les teaches that the priest could not take the holy utensils home for personal use, because they were set apart only for God's purposes. In the same way, believers have been set aside and are intrinsically in God's program to be used as He sees fit, because they are His. That means we should be holy, or set apart, and without blame before Him. Les acknowledges that every believer is a sinner saved by grace, still has the old sin nature, and still sins, yet God says we are blameless. He also notes that the Corinthians were carnal, not spiritual, saved out of paganism, and had not come to understand the position in the heavenlies; Paul writes to them in I Corinthians 1:6-7.