In the First Letter to the Thessalonians, Paul makes an exhortation to live in holiness:

It is God's will that you should be sanctified
1 Thessalonians 4:3

In theological terms, Paul is speaking about the life in God's grace, that is, the righteous life: Do to others as you would have them do to you, Matthew 7:12. We sin or live righteously (Thomas Aquinas), sanctity is something more than a righteous life, holiness is the fruit of the "constancy"(spiritual gift of Fear of the Lord) in "righteousness". To understand better holiness one must understand the concepts of image and likeness.
The Image of God is given in the Holiness. Rememeber that in Metaphysics  the definition of the Image is perfect likeness. And the likeness is given in the essence.
Example: A man is like another as they share the essence of reason. However they can not be the image of each other, the image is given in the twins.
Likeness to God as such is given in the charity, the good love, the love for the good. In the good love we received the seven gifts of the state of grace, the completitude.
The image is given in the fruits of the spirit, the fruits of the spirit define the character of God, the fruits can also be interpreted as the ultimate goals of life. . After reaching the likeness through the Gnosis (receiving the holy spirit in the Catholic Religion), the fruits of the spirit gradually emerge through the contemplative activity.
The difference between likeness and image is the same as that between a sage and a saint, not every wise is image, but it is necessarity that who is image is also likeness.
The nature of man is the conformity to God. And in this conformity we imitate (golden rule), we emulate (gift of conuncil), we are likeness (Charity) and we are image (Fruits of the spirit).