"Not by what we say but by what we do"

Via Kathryn Jean Lopez at NRO. Gregory of Nyssa (4th c.):

When we consider that Christ is the true light, having nothing in common with deceit, we learn that our own life also must shine with the rays of that true light. Now these rays of the Sun of Justice are the virtues which pour out to enlighten us so that we may put away the works of darkness and walk honorably as in broad daylight. When we reject the deeds of darkness and do everything in the light of day, we become light and, as light should, we give light to others by our actions.

If we truly think of Christ as our source of holiness, we shall refrain from anything wicked or impure in thought or act and thus show ourselves to be worthy bearers of his name. For the quality of holiness is shown not by what we say but by what we do in life.