Excerpt from The Life of St. Antony:

'Wherefore having already begun and set out in the way of virtue, let us strive the more that we may attain those things that are before. And let no one turn to the things behind, like Lot's wife, all the more so that the Lord hath said, "No man, having put his hand to the plough, and turning back, is fit for the kingdom of heavens ." And this turning back is nought else but to feel regret, and to be once more worldly-minded. But fear not to hear of virtue, nor be astonished at the name. For it is not far from us, nor is it without ourselves, but it is within us, and is easy if only we are willing. That they may get knowledge, the Greeks live abroad and cross the sea, but we have no need to depart from home for the sake of the kingdom of heaven, nor to cross the sea for the sake of virtue. For the Lord aforetime hath said, "The kingdom of heaven is within you.'' Wherefore virtue hath need at our hands of willingness alone, since it is in us and is formed from us. For when the soul hath its spiritual faculty in a natural state virtue is formed. And it is in a natural state when it remains as it came into existence. And when it came into existence it was fair and exceeding honest. For this cause Joshua, the son of Nun, in his exhortation said to the people," Make straight your heart unto the Lord God of Israel," and John, "Make your paths straight." For rectitude of soul consists in its having its spiritual part in its natural state as created. But on the other hand, when it swerves and turns away from its natural state, that is called vice of the soul. Thus the matter is not difficult. If we abide as we have been made, we are in a state of virtue, but if we think of ignoble things we shall be accounted evil. If, therefore, this thing had to be acquired from without, it would be difficult in reality; but if it is in us, let us keep ourselves from foul thoughts. And as we have received the soul as a deposit, let us preserve it for the Lord, that He may recognise His work as being the same as He made it.