There's a whole book waiting to be written on the transmutation of the Roman ideals of war and duty to the later chivalric ideals. I think the changes in the concept of virtus alone (say for example to look at the handful of chivalric texts actually written in Latin) are incredibly complex and fascinating.

...see, now my mind is running off on a whole tangent on sacrifice and Roman sacrifice versus medieval sacrifice because of the introduction of Christianity and...

...wait, this post was about Supernatural?