Luke 17:6

Or this is said because faith keeps out the unclean spirit, especially since the nature of the tree falls in with this meaning. For the fruit of the mulberry is at first white in the blossom, and being formed from thence grows red, and blackens as it gets ripe. The devil also having by transgression fallen from the white flower of the angelic nature and the bright beams of his power, grows terrible in the black odour of sin.

(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.

(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.

The disciples hearing our Lord discoursing of certain arduous duties, such as poverty, and avoiding offences, entreat Him to increase their faith, that so they might be able to follow poverty, (for nothing so prompts to a life of poverty as faith and hope in the Lord,) and through faith to guard against giving offences. Therefore it is said, And the Apostles said unto the Lord, Increase our faith. But our Lord told them that they asked well, and that they ought to believe stedfastly, forasmuch as faith could do many things; and hence it follows, And the Lord said, If ye had faith as a grain of mustard seed, &c. Two mighty acts are here brought together in the same sentence; the transplanting of that which was rooted in the earth, and the planting thereof in the sea, (for what is ever planted in the waves?) by which two things He declares the power of faith.