"If we wish to understand the power of Christ’s blood, we should go back to the ancient account of its prefiguration in Egypt. Sacrifice a lamb without blemish, commanded Moses, and sprinkle its blood on your doors. If we were to ask him what he meant, and how the blood of an irrational beast could possibly save men endowed with reason, his answer would be that the saving power lies not in the blood itself, but in the fact that it is a sign of the Lord’s blood. In those days, when the destroying angel saw the blood on the doors he did not dare to enter, so how much less will the devil approach now when he sees, not that figurative blood on the doors, but the true blood on the lips of believers, the doors of the temple of Christ.
If you desire further proof of the power of this blood, remember where it came from, how it ran down from the cross, flowing from the Master’s side. The gospel records that when Christ was dead, but still hung on the cross, a soldier came and pierced his side with a lance and immediately there poured out water and blood. Now the water was a symbol of baptism and the blood, of the holy eucharist. The soldier pierced the Lord’s side, he breached the wall of the sacred temple, and I have found the treasure and made it my own. So also with the lamb: the Jews sacrificed the victim and I have been saved by it."
~From the Catecheses by Saint John Chrysostom, bishop (AD 344-407)