The Mass makes present and effective for us, right here and now, the Victim on the altar of the cross. The death of Jesus is more than a mere fact of history. It is an eternal sacrifice. There are no yesterdays with God. In the infinite Mind of God, to whom all things past are present, Jesus hangs eternally upon the cross.
It is not an easy truth to grasp, but it is the truth: that at Mass time and distance are annihilated in a mystical sense; and you and I stand beneath the cross as the Son of God offers himself in sacrifice for us.
…Jesus does not multiply himself so that there are many Jesuses; nor does he divide himself up among the many hosts. There is but one Jesus, whole and undivided. His multilocation is not the result of multiplication or division; it is the result of the suspension of the laws of space as far as his sacred body is concerned. It is as though he himself stayed in one place, and all parts of space were brought to him, were pinpointed in him. It is easy to see why the Holy Eucharist is called—and is—the sacrament of unity. When we receive Holy Communion we are where Jesus is; we and our fellow communicants all over the world. Space has dissolved for us, and we are there together—one in Christ.
—from The Faith Explained by Leo J. Trese, Ch. 27: The Mass and Ch. 26: The Eucharist