We undertook, in the name of the Lord, and promised to you, beloved, to treat of that grace and truth of God, full of which the only-begotten Son, our Lord and Saviour Jesus Christ, appeared to the saints, and to show how, as a matter belonging to the New Testament, it is to be distinguished from the Old Testament. Give, then, your attention that what I receive in my measure from God you in your measure may receive and hear the same. For it will only remain if, when the seed is scattered in your hearts, the birds take it not away, nor thorns choke it, nor heat scorch it, and there descend upon it the rain of daily exhortations and your own good thoughts, by which that is done in the heart which in the field is done by means of harrows, so that the clod is broken, and the seed covered and enabled to germinate: that you bear fruit at which the husbandman may be glad and rejoice. But if, in return for good seed and good rain, you bring forth not fruit but thorns, the seed will not be blamed, nor will the rain be in fault; but for thorns due fire is prepared.
Source: Tractates on the Gospel of John (New Advent)