Purify - Hebrews 9:13-14

Hebrews 9:13-14 ESV 

For if the blood of goats and bulls, and the sprinkling of defiled persons with the ashes of a heifer, sanctify for the purification of the flesh, how much more will the blood of Christ, who through the eternal Spirit offered himself without blemish to God, purify our conscience from dead works to serve the living God.

The sacrificial blood of animals only served to sanctify the flesh of man, but did not make him clean on the inside. The flesh could be purified, but the soul was not. Repentance was not required. 

The sacrificial blood of Christ would cleanse the whole man. He began to preach a different word. "Repent." This allows for a cleansing of the inside, because it's what's on the inside of a man that defiles Him. Repentance will make one pure, and only the pure in heart will see God. 

A pure heart is a choice, an ongoing choice. He is faithful even when we are not.