"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."
-St. John Chrysostom, On The Power of Christ's Blood
Mobile being is divided according to act and potency. Potency, again, when speaking of mobile being, is regarded as matter or the material cause. Act is the formal cause, rendering the being in actuality and intelligibility. Potency, though, according to its own power, cannot become act. It must therefore become act by the operation of another. This other, which must itself be in act in some way in order to exercise power over that which is, of itself, in potency, is the efficient cause. No being, though, operates without some determinate end, all potency is ordered toward some act. This end is the final cause, that for the sake of which a potency becomes an actuality. So, from this division of mobile being into act and potency, we derive the four causes.