On Baptism by Tertullian
Chapter VII.--Of the Unction.
After this, when we have issued from the font, we are thoroughly anointed with a blessed unction, -- (a practice derived) from the old discipline, wherein on entering the priesthood, men were wont to be anointed with oil from a horn, ever since Aaron was anointed by Moses. Whence Aaron is called |Christ,| from the |chrism,| which is |the unction;| which, when made spiritual, furnished an appropriate name to the Lord, because He was |anointed| with the Spirit by God the Father; as written in the Acts: |For truly they were gathered together in this city against Thy Holy Son whom Thou hast anointed.| Thus, too, in our case, the unction runs carnally, (i.e. on the body,) but profits spiritually; in the same way as the act of baptism itself too is carnal, in that we are plunged in water, but the effect spiritual, in that we are freed from sins.