From the WCT Holiness of Heart found in Jeremiah 31:
In Jer. God promises a new covenant. God will write Gods law on peoples hearts. Then truly I will be their God, and they shall be my people (Jer 31:31, 33).
Holiness of heart, not mere ceremonial holiness, has always been Gods goal for his people. God intends inner transformation the cleansing and liberating of the inmost springs of action and motivation. Jesus emphasized that evil attitudes and actions come from within (Mark 7:21-23). It is the heart, therefore, that must be cleansed.
Wesley saw that holiness of heart could be misunderstood, however, as solely an interior change. The outward ethical dimension so prominent in Scripture could be missed. So Wesley often used the phrase inward and outward (or all inward and outward) holiness to forestall any disconnection between inner transformation and outward behavior. Holiness of heart means transformation by Gods grace, enabling people to be holy, loving, and Christ-like in their relationships with one another and with the land.
https://johnmeunier.wordpress.com/2010/05/03/wesley-study-bible-on-holiness/