The ark--the resting place for the Lord-- in later times would reside in the temple, but had also accompanied the army into battle. The Lord--the creator of the earth (vv 1-2), the holy one.
Who can approach the holy? Still a question for modern worshipers.
The clean hands and pure hearts--are these absolute requirements of who can get in, or are they strong suggestions for how worshipers should order their lives, or they necessary in order for us to receive blessing--or to recognize what is a blessing?