Gregory the Great points out that to produce the ‘blade’ is to hold the first beginnings of what is good. It arrives at maturity when virtue conceived in the mind leads to advancement in good works. The ‘full corn’ fructifies in the ‘ear’ when virtue makes such great progress that it comes into its fullest possible expression.
Clement of Alexandria tells us that the word which proclaims the kingdom of Heaven is sharp and pungent as mustard It represses bile (anger) and checks inflammation (pride). From this word flows the soul’s true vitality and fitness for eternity. To such increased size did the growth of the word come that the tree which sprang from it – the Church of Christ – filled the world, so that the birds of the air (that is holy angels and lofty souls) dwelt in its branches.
St Ambrose commented that the unbruised seed was of little value, whereas, once bruised, its full power comes forth. So, faith is a simple thing, but if bruised by its enemies it gives forth proof of its power. The Lord Himself is the mustard seed, and the attempt to crush him brought forth his full power to save all who believe in Him.
Tertullian explains that Christ do clearly declared who he was during his mission, that it was clear to his holy Apostles who he was, and that is what they have proclaimed to the world