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 (that is the Church of Christ now being established over the whole earth) filled the world, so that the birds of the air (that is, holy angels and lofty souls) dwelt in its branches.
Source: Nicetas Bishop of Heraclea