3 If, my brethren, we understand by heaven the firmament which we see with our bodily eyes, we shall indeed so err, as to imagine that we cannot ascend there without ladders, or some scaling machines: but if we ascend spiritually, we ought to understand heaven spiritually: if the ascent be in affection, heaven is in righteousness. What is then the heaven of God? All holy souls, all righteous souls. For the Apostles also, although they were on earth in the flesh, were heaven; for the Lord, enthroned in them, traversed the whole world.
He then dwells in heaven. How?...How long are they the temple according to faith? As long as Christ dwells in them through faith; as the Apostle says, “That Christ may dwell in your hearts through faith.” But they are already heaven in whom God already dwells visibly, who see Him face to face; all the holy Apostles, all the holy Virtues, Powers, Thrones, Lordships, that heavenly Jerusalem, wanderers from whence we groan, and for which we pray with longing; and there God dwells.
Thither has the Psalmist lifted up his faith, there he rises in affection, with longing hopes: and this very longing causes the soul to purge off the filth of sins, and to be cleansed from every stain, that itself also may become heaven; because it has lifted up its eyes unto Him who dwells in heaven. For if we have determined that that heaven which we see with our bodily eyes is the dwelling of God, the dwelling of God will pass away; for “heaven and earth will pass away.” Then, before God created heaven and earth, where did He dwell?
But some one says: and before God made the Saints, where did He dwell? God dwelt in Himself, he dwelt with Himself, and God is with Himself. And when He deigns to dwell in the Saints, the Saints are not the house of God in such wise, as that God should fall when it is withdrawn. For we dwell in a house in one way, in another way God dwells in the Saints. You dwell in a house: if it be withdrawn, you fall, but God so dwells in the Saints, that if He should Himself depart, they fall...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)