3 Of this ax does John speak, saying, “Now is the ax laid unto the root of the trees; every tree which brings not forth good fruit shall be hewn down, and cast into the fire.” With this ax does the Householder in the Gospel threaten, saying, “Behold these three years I come to this tree, and find no fruit on it.” Now I must clear the ground; wherefore let it be cut down. And the husbandman intercedes, saying, “Lord, let it alone this year also, till I shall dig about it and dung it; and if it bear fruit, well; and if not, then You shall come and cut it down.” So the Lord has visited mankind as it were three years, that is, at three several times.
The first time was before the Law; the second under the Law; the third is now, which is the time of grace. For if He did not visit mankind before the Law, whence was Abel, and Enoch, and Noe, and Abraham, and Isaac, and Jacob, whose Lord He was pleased to be called? And He to whom all nations belonged, as though He were the God of three men only, said, “I am the God of Abraham, and Isaac, and Jacob.” But if He did not visit under the Law, He would not have given the Law itself.
After the Law, came the very Master of the house in person; He suffered, and died, and rose again; He gave the Holy Spirit, He made the Gospel to be preached throughout all the world, and yet a certain tree remained unfruitful. Still is there a certain portion of mankind, which does not yet amend itself. The husbandman intercedes; the Apostle prays for the people; “I bow my knees,” he says, “unto the Father for you, that being rooted and grounded in love, you may be able to comprehend with all saints what is the breadth, and length, and depth, and height; and to know the love of Christ which passes knowledge, that you might be filled with all the fullness of God.” By bowing the knees, he intercedes with the Master of the house for us, that we be not rooted up.
Therefore since He must necessarily come, let us take care that He find us fruitful. The digging about the tree is the lowliness of the penitent. For every ditch is low. The dunging it, is the filthy robe of repentance. For what is more filthy than dung; yet if well used, what more profitable?
Source: Sermons on Selected Lessons of the New Testament (New Advent)