In Ascetical Homilies III/8, Isaac tackles the question of the mode in which God's saints are set aside and sanctified through prayer. In the opening of the homily, he describes the prayerful person as a kind of temple of God in which heaven is present:
God's temple is a house of prayer. The soul in which a constant remembrance of God is sanctified is likewise a house of prayer. If all the saints are sanctified by the Spirit so that they may be temples of the worshiped Trinity, it is because the Holy Spirit sanctifies them keeping alive in them the remembrance of God. For constant prayer consists in a constant remembrance of God (III/8, 1).
He quotes an unidentified line from an earlier father: Always remember God and your mind will be heaven (ibid.).
For Isaac, then, the way in which we are sanctified and made worthy temples of the Holy Trinity is through a constant remembrance of God. This is something that the Holy Spirit accomplishes in us, it is the way God makes us to be holy. The emphasis here on the transformation of the mind of the believer is reminiscent of Paul's words to the Romans: be transformed by the renewing of your minds (Rom 12.2).
Importantly for Isaac, too, this constant remembrance of God is a way of battling and dispersing temptations and passions. This is because these things are not present in heaven; and if by the remembrance of God we are made temples of God and our minds made a small corner of heaven, then there may not be any temptations or passions either:
So then if through constant prayer our soul becomes another heaven, then in this heaven there is no good thing lacking, this heaven is not trodden by any evil and no temptation comes near to it either, nor the passions of the body and of the spirit, nor a remembrance of bad things, nor any other thing which troubles the body, nor darkness or the misfortunes of the soul. If these things are present, this happens because we wander and distance ourselves from the remembrance o God, and for that reason we wander lost, falling into all manner of evils (III/8, 2).
The transformation of the mind and its focus upon God consequently is a powerful thing for Isaac. If we suffer from passions and sins and temptations and all manner of evil, it is because we have drifted from the contemplation of God and the remembrance of his things. On the other hand, if we remain in this state of meditation upon the LORD, we make our minds into a small heaven, we fashion ourselves into worthy temples of God, and we are sanctified by the Holy Spirit into small spots of heaven on earth.
Consequently Isaac enjoins his listeners and readers to diligence in meditation: Let us strive and struggle, therefore, in prayer, which is the brilliant form of the remembrance of the Lord our God; and all the temptations which the Providence [of God] permits to be sent to disturb in us this remembrance o God will depart from us, due to the blaze of prayer and the crucifixion of the intellect which is accomplished in it. . . . Therefore when we strive in [prayer] and make room in us for the remembrance of the Lord through our ceaseless prayer towards him, then the temptations will depart, passions will be calmed, Satan is cast out, sadness no longer finds its place with us, our sorrows are soothed, and everything which opposes us departs, leaving room for the remembrance of God which we earlier enjoyed; they flee and perish before their Lord (III/8, 3-4).
Perhaps this is the reason why the Bible teaches us that the blessed man meditates on the law of the LORD day and night (Ps 1.2) -- because through this constant remembrance of God he is transformed into a worthy temple of God!