Hebrews 3:6
8 There is no persecution now, and God grant there may never be: but there is another war, that of the desire of money, of envy, of the passions. Paul, describing this war, says, “We wrestle not against flesh and blood.” This war is ever at hand. Therefore he wishes us to stand ever armed. Because he wishes us to stand ever armed, he says, “Stand, having girded yourselves about.” Which itself also belongs to the time present, and expresses that we ought ever to be armed. For great is the war through the tongue, great that through the eyes; this then we must keep down— great [too] is that of the lusts.
Therefore he begins at that point to arm the soldier of Christ: for “stand,” says he, “having your loins girt about,” and he added “with truth.” Why “with truth”? Because lust is a mockery and a lie: wherefore the prophet says, “My loins are filled with mockings.” The thing is not pleasure, but a shadow of pleasure. “Having your loins,” he says, “girt about with truth”; that is, with true pleasure, with temperance, with orderly behavior. For this cause he gives this advice, knowing the unreasonableness of sin, and wishing that all our members should be hedged round; for “unjust anger” it is said, “shall not be guiltless.”
Moreover he wishes us to have around us a breastplate and a buckler. For desire is a wild beast which easily springs forth, and we shall have need of walls and fences innumerable, to overcome, and to restrain it. And for this cause God has built this part [of our body] especially with bones, as with a kind of stones, placing around it a support, so that [desire] might not at any time, having broken or cut through, easily injure the whole man. For it is a fire (it is said) and a great tempest, and no other part of the body could endure this violence. And the sons of the physicians too say that for this cause the lungs have been spread under the heart, so that the heart being itself [put] into something soft and tender, by beating as it were into a sort of sponge, may continually be rested, and not [by striking] against the resisting and hard sternum, receive hurt through the violence of its beatings. We have need therefore of a strong breastplate, so as to keep this wild beast always quiet.
We have need also of an helmet; for since the reasoning faculty is there, and from this it is possible for us either to be saved, when what is right is done, or it is possible for us to be ruined— therefore he says, “the helmet of salvation.” For the brain is indeed by nature tender, and therefore is covered above with the skull, as with a kind of shell. And it is to us the cause of all things both good and evil, knowing what is fitting, or what is not so. Yea and our feet too and our hands need armor, not these hands, nor these feet, but as before those of the soul— the former by being employed about what is right, the latter, that they may walk where they ought. Thus then let us thoroughly arm ourselves, and we shall be able to overcome our enemies, and to wreathe ourselves with the crown in Christ Jesus our Lord, with whom to the Father together with the Holy Ghost be glory, might, honor, now and for ever and world without end. Amen.
Source: Homilies on the Epistle to the Hebrews (New Advent)