From "The Responses of Pope St. Nicholas I to the Questions of the Bulgars" (Letter 99), Chapter LVIIII, A.D. 866:
"We consider what you asked about pants (femoralia) to be irrelevant; for we do not wish the exterior style of your clothing to be changed, but rather the behavior of the inner man within you, nor do we desire to know what you are wearing except Christ — for however many of you have been baptized in Christ, have put on Christ [Gal. 3:27] — but rather how you are progressing in faith and good works. But since you ask concerning these matters in your simplicity, namely because you were afraid lest it be held against you as a sin, if you diverge in the slightest way from the custom of other Christians, and lest we seem to take anything away from your desire, we declare that in our books, pants (femoralia) are ordered to be made, not in order that women may use them, but that men may. But act now so that, just as you passed from the old to the new man, [cf. Eph. 4:22-24; Col. 3:9-10] you pass from your prior custom to ours in all things; but really do what you please. For whether you or your women wear or do not wear pants (femoralia) neither impedes your salvation nor leads to any increase of your virtue.
"Of course, because we have said that pants are ordered to be made, it should be noted that we put on pants spiritually, when we restrain the lust of the flesh through abstinence; for those places are constrained by pants in which the seats of luxury are known to be. This is why the first humans, when they felt illicit motions in their members after sin, ran into the leaves of a fig tree and wove loin cloths for themselves.[cf. Gen. 3:7] But these are spiritual pants, which you still could not bear, and, if I may speak with the Apostle, you are not yet able; for you are still carnal.[I Cor. 3:2] And thus we have said a few things on this matter, although, with God's gift, we could say many more."