Luke 17:5

Or this number is used because God rested on the seventh day from His works. After the seventh day of the world everlasting rest is promised us, that as the evil works of that world shall then cease, so also may the sharpness of punishment be abated.

(Hom. 57. in Matt.) He mentions the mustard seed, because, though small in size, it is mightier in power than all the others. He implies then that the least part of faith can do great things. But though the Apostles did not transplant the mulberry tree, do not thou accuse them; for our Lord said not, You shall transplant, but, You shall be able to transplant. But they did not, because there was no need, seeing that they did greater things. (Hom. 32 in 1 ad Cor. c. 13:2.). But some one will ask, How does Christ say, that it is the least part of faith which can transplant a mulberry tree or a mountain, whereas Paul says that it is all faith which moves mountains? We must then answer, that the Apostle imputes the moving of mountains to all faith, not as though only the whole of faith could do this, but because this seemed a great thing to carnal men on account of the vastness of the body. The mulberry may be also compared to the devil, for as by the leaves of the mulberry tree certain worms are fed, so the devil, by the imaginations which proceed from him, is feeding for us a never dying worm; but this mulberry tree faith is able to pluck out of our souls, and plunge it into the deep.

(de Quæst. Ev. lib. 2. qu. 39.) We may indeed understand that they asked for the increase of that faith by which men believe in the things which they see not; but there is further signified a faith in things, whereby not with the words only, but the things themselves present, we believe. And this shall be, when the Wisdom of God, by whom all things were made, shall reveal Himself openly to His saints face to face.

(22. Mor. c. 21.) That is, that the faith which has already been received in its beginning, might go on increasing more and more unto perfection.

Or our Lord here compares perfect faith to a grain of mustard seed, because it is lowly in appearance, but fervid in heart. But mystically by the mulberry tree, (whose fruit and branches are red with a blood-red colour,) is represented the Gospel of the cross, which, through the faith of the Apostles being uprooted by the word of preaching from the Jewish nation, in which it was kept as it were in the lineal stock, was removed and planted in the sea of the Gentiles.