In Matthew 13, we hear Our Lord compare the kingdom of heaven to a grain of mustard seed. Later, in Matthew 17, He compares faith to the grain of mustard. From these two comparisons, St Ambrose draws the following: “If the kingdom of heaven is like to a grain of mustard seed, and faith is like to a grain of mustard seed, faith is then truly the kingdom of heaven, and the kingdom of heaven is faith. He therefore that has faith possesses the kingdom of heaven.”
The Gospel, then, teaches us that faith and the kingdom of heaven are intimately related. Of course, one needs faith to enter the kingdom of heaven. But also, the kingdom of heaven is seen only with the eyes of faith. In other words, the kingdom we strive to attain promises security, riches and other ‘rewards’ which can be grasped and held dear only by faith. Faith, of course, is not readily apparent or visible, and in fact seems as insignificant as the mustard seed. In the same way, the kingdom of heaven is not readily apparent or visible, and seems insignificant.
Consider also this: the full power of the mustard seed is revealed only when it is crushed. When nature crushes the seed, a great tree takes root. When a man deliberately crushes the mustard seed, the seed produces a strong and rich spice. In the same way, crushing faith by persecution or martyrdom reveals both its strength and the ‘spice’ of overwhelming love for God. And ‘crushing’ or breaking open the kingdom of heaven reveals both the riches it contains, and the love it releases.
At root, then, what is truly the kingdom of heaven? Once again, St Ambrose urges us to believe that “The Lord Himself is the grain of mustard seed. He was without injury; but the people were unaware of Him. … [Yet] He chose to be crushed. … He chose to be planted in the earth [when He was buried]. … [Then] He sprung up in a garden, where He also rose from the dead, and became a tree;” namely, the Tree of Life.