To guard the mind requires that we know Christ's presence in our heart. Once we know this presence, we can bring our innocent thoughts to Christ. But first, our heart needs to be opened. Until it is open, Fr. Dimitru Staniloae says that we must knock at its door, with thoughts sacrificed to Christ, with the hope that we will gain the awareness of His presence and by this our heart will be opened.
He also says that we don't have a full feeling of His presence at first. We will experience gradual progress in this. We must be persistent and have patience.
The whole notion of guarding the mind is dependent on us being able to bring our thoughts to the door of the heart. Therefore it is also called watching of the heart.
Standing watch at the door of the heart, the mind does nothing but keep itself from going astray, because the heart is, after all, nothing but the depths of the mind.
Think about how often our minds go astray. How often by our immersion in our ego needs we ignore this place of the heart. The mind never stops and our actions seemingly spinout of the control of our highest values. We need to be ever vigilant.
Saint Mark the Ascetic says,
The mind must keep vigil over the heart and guard it with all watchfulness, trying to penetrate into its innermost and undisturbed chamber, where there are no winds of evil thoughts... to be vigilant over the heart and go ever deeper into it and to approach God alone, without becoming disgusted with the toils of attention and persistence.
