6 “And as you go,” says He, “preach, saying, The kingdom of Heaven is at hand.”
Do you see the greatness of their ministry? Do you see the dignity of apostles? Of nothing that is the object of sense are they commanded to speak, nor such as Moses spoke of, and the prophets before them, but of some new and strange things. For while the former preached no such things, but earth, and the good things in the earth, these preached the kingdom of Heaven, and whatever is there.
And not from this circumstance only were these the greater, but also from their obedience: in that they shrink not, nor are they backward, like those of old; but, warned as they are of perils, and wars, and of those insupportable evils, they receive with great obedience His injunctions, as being heralds of a kingdom.
“And what marvel,” says one, “if having nothing to preach that is dismal or grievous, they readily obeyed?” What do you say? Nothing grievous enjoined them? Do you not hear of the prisons, the executions, the civil wars, the hatred of all men? All which, He said a little while after, they must undergo. True, as to other men, He sent them to be procurers and heralds of innumerable blessings: but for themselves, He said and proclaimed beforehand, that they were to suffer terrible and incurable ills.
After this, to make them trustworthy, He says,
“Heal the sick, cleanse the lepers, cast out devils: freely you have received, freely give.”
See how He provides for their conduct, and that no less than for their miracles, implying that the miracles without this are nothing. Thus He both quells their pride by saying, “Freely you have received, freely give;” and takes order for their being clear of covetousness. Moreover, lest it should be thought their own work, and they be lifted up by the signs that were wrought, He says, “freely you have received.” “You bestow no favor on them that receive you, for not for a price did ye receive these things, nor after toil: for the grace is mine. In like manner therefore give ye to them also, for there is no finding a price worthy of them.”
Source: Homilies on the Gospel of St. Matthew (New Advent)