Holiness in humans is single-hearted devotion to God through our faith in Christ Jesus as Lord.  This definition reflects Jesus’ identification of the greatest of all the commandments:

The most important commandment is this: ‘The Lord our God is the one and only Lord. And you must love the Lord your God with all your heart, all your soul, all your mind, and all your strength.’ The second is equally important: ‘Love your neighbor as yourself.’ No other commandment is greater than these (Mark 12:29-31, NLT)