Irenaeus of Lyons:
To save us from forfeiting life by losing the Spirit who possesses us, and to exhort us to share in the Spirit, the Apostle declares that it is impossible for flesh and blood to gain possession of the kingdom of God.
In other words: Do not deceive yourselves, for if the Word of God and the Spirit of the Father do not dwell in you, and you lead a life of vanity and carelessness, as though you were merely flesh and blood, you cannot obtain the kingdom of God.
He says this to prevent us from indulging our physical nature and spurning the grafting of the spirit, for when you were a wild olive, he says, you were grafted into the cultivated olive and came to share that olive’s rich sap.
With a grafted wild olive, if the graft fails to take it is cut off and thrown into the fire, but if the graft is successful and the wild olive takes on the qualities of the cultivated one, it develops into a fruitful olive, like one planted in a royal garden.
In the same way we too, if faith leads us to reform and we receive the Spirit of God and produce the fruit of that Spirit, we too shall be spiritual, as if planted in the garden of God.
But if we spurn the Spirit and remain what we were before, choosing to belong to the flesh rather than the Spirit, we would rightly be told: It is impossible for flesh and blood to gain possession of the kingdom of God: in other words, no wild olive will be admitted into God’s garden.
Thus in comparing flesh and blood with the wild olive, the Apostle has given us a wonderful picture of our nature and of the whole of God’s plan for us. The olive, if neglected and left to grow for a time in some deserted spot, becomes a wild olive again and produces poor fruit, but if it is taken care of once more and grafted, it returns to its earlier fertility.
And it is the same with us. We too may become careless and allow the bad fruit of worldly desires to grow in us, and by our own fault fail to bear the fruits of righteousness. While we sleep the enemy sows weeds, which is why the Lord commanded his disciples to be watchful.
If, however, when we have become barren of righteousness and as it were entangled in brambles, we are cared for again and receive the word of God like a graft, we then return to our original nature, the nature created in the image and likeness of God.
Irenaeus of Lyons (2nd century AD – c. 202):Adversus Haereses, 5.9.4-10 (SC 153:122-6); ; from the Monastic Office of Vigils, Saturday of the Third Week of Ordinary Time, Year I.
