3 But beware of supposing this to be plain ointment. For as the Bread of the Eucharist, after the invocation of the Holy Ghost, is mere bread no longer, but the Body of Christ, so also this holy ointment is no more simple ointment, nor (so to say) common, after invocation, but it is Christ's gift of grace, and, by the advent of the Holy Ghost, is made fit to impart His Divine Nature. Which ointment is symbolically applied to your forehead and your other senses; and while your body is anointed with the visible ointment, your soul is sanctified by the Holy and life-giving Spirit.
Source: Catechetical Lectures (New Advent)