The first is that by which as far as the body is concerned we make light of all the wealth and goods of this world;
The second, that by which we reject the fashions and vices and former affections of soul and flesh;
The third, that by which we detach our soul from all present and visible things, and contemplate only things to come, and set our heart on what is invisible.
And we read that the Lord charged Abraham to do all these three at once, when He said to him “Get thee out from thy country, and thy kinsfolk, and thy father’s house” (Gen. 12:1). First He said “from thy country,” i.e., from the goods of this world, and earthly riches;
Secondly, “from thy kinsfolk,” i.e., from this former life and habits and sins, which cling to us from our very birth and are joined to us as it were by ties of affinity and kinship;
Thirdly, “from thy father’s house,” i.e., from all the recollection of this world, which the sight of the eyes can afford.
[...] And this happens when being dead with Christ to the rudiments of this world, we no longer, as the Apostle says, regard “the things which are seen, but those which are not seen, for the things which are not seen are eternal” (2 Cor. 4:18).
Instead, going forth in heart from this temporal and visible home, we turn our eyes and heart towards that in which we are to remain for ever.
And this we shall succeed in doing when, while we walk in the flesh, we are no longer at war with the Lord according to the flesh, proclaiming in deed and actions the truth of that saying of the blessed Apostle “Our conversation is in heaven” (Phil. 3:20).
To these three sorts of renunciations the three books of Solomon suitably correspond:
For Proverbs answers to the first renunciation, as in it the desires for carnal things and earthly sins are repressed;
To the second Ecclesiastes corresponds, as there everything which is done under the sun is declared to be vanity;
To the third the Song of Songs, in which the soul soaring above all things visible, is actually joined to the word of God by the contemplation of heavenly things.
John Cassian (c. 360-435): Conferences 3,6.