1 Chronicles 15:12, 14 And said unto them, Ye are the chiefs of the fathers of the Levites: sanctify yourselves, both ye and your brethren, that ye may bring up the ark of the Lord God of Israel unto the place that I have prepared for it….So the priests and the Levites sanctified themselves to bring up the ark of the Lord God of Israel.
David gathered all Israel together in Jerusalem to bring up the ark of the Lord to the place which he had prepared for it (the temple was not yet built). The priests and the Levites were to carry the ark and go before it, but before bringing it up they had to sanctify themselves. And they did.
The Presence of God was coming into their midst with much music and great celebration. A special place was prepared for the Holy Presence of the living God. And those who were charged with bringing it up were told to sanctify themselves, for this was a holy thing and a solemn event. And no unclean or common sinner could touch the holy ark, like Uzzah, and live. No human can approach a Holy God, or attend the wedding feast, without a robe of righteousness, lest he die.
Therefore, the priests were to become clean and pure before coming near the Holy Presence. They made themselves holy by going through the proper rituals, washing themselves, and offering sacrifices, according to the Law. All of this was a symbol and a prophetic foreshadowing of the Messiah, who would make us holy.
Jesus has fully accomplished all the ritual cleansing, washing, and sacrificing for us. We prepare our hearts by confession, repentance, believing Jesus, and receiving forgiveness. The Holy Spirit applies that Gospel to our hearts and makes us holy so that may touch God. God touches a heart sanctified by the Word and the Spirit. Then, like at Christmas, a holy God comes into a prepared heart.