From a homily on Matthew by Saint John Chrysostum, bishop

As long as we are sheep, we overcome and, though surrounded by countless wolves, we emerge victorious; but if we turn into wolves, we are overcome, for we lose the shepherd’s help. He, after all, feeds the sheep not wolves, and will abandon you if you do not let him show his power in you.

What he says is this: “Do not be upset that, as I send you out among the wolves, I bid you be as sheep and doves. I could have managed things quite differently and sent you, not to suffer evil nor to yield like sheep to the wolves, but to be fiercer than lions. but the way I have chosen is right. It will bring you greater praise and at the same time manifest my power.” That is what he told Paul: My grace is enough for you, for in weakness my power is made perfect. “I intend,” he says, “to deal the same way with you.” For, when he says, I am sending you out like sheep, he implies: “But do not therefore lose heart, for I know and am certain that no one will be able to overcome you.”

The Lord, however, does want them to contribute something, lest everything seem to be the work of grace, and they seem to win their reward without deserving it. Therefore he adds: You must be clever as snakes and innocent as doves. But, they may object, what good is our cleverness amid so many dangers? How can we be clever when tossed about by so many waves? However great the cleverness of the sheep as he stands among the wolves – so may wolves! – what can it accomplish? However great the innocence of the dove, what good does it do him, with so many hawks swooping upon him? To all this I say: Cleverness and innocence admittedly do these irrational creatures no good, but they can help you greatly.

What cleverness is the Lord requiring here? The cleverness of a snake. A snake will surrender everything and will put up no great resistance even if its body is being cut in pieces, provided it can save its head. So you, the Lord is saying, must surrender everything but your faith: money, body, even life itself. For faith is the head and the root; keep that, and though you lose all else, you will get it back in abundance. The Lord therefore counseled the disciples to be not simply clever or innocent; rather he joined the two qualities so that they become a genuine virtue. He insisted on the cleverness of the snake so that deadly wounds might be avoided, and he insisted on the innocence of the dove so that revenge might not be taken on those who injure or lay traps for you. Cleverness is useless without innocence.

Do not believe that this precept is beyond you power. More than anyone else, the Lord knows the true natures of created things; he knows that moderation, not a fierce defense, beats back a fierce attack.