John 21:3

How comes it that Peter, after his conversion, should return to his fishing, when Jesus Christ had said, that he that sets his hand to the plough, and looks back, is not worthy of the kingdom of heaven? The employments they applied to before their conversion, without being guilty of sin, these they might, without fault, exercise, after their conversion: therefore Peter returned to his fishing; but St. Matthew never returned to his custom-house, because when once converted, we never can be allowed to give ourselves to these employments, which of themselves lead to sin. And there are many pursuits which can scarcely, or not at all, be followed without sin. (St. Gregory, hom. xxiv. in Evan.)