“But Jesus, knowing in himself that his disciples murmured at it, said to them, "Do you take offense at this? Then what if you were to see the Son of man ascending where he was before?"”
Short answer
The Church holds both truths together. Christ ascended in his body and is seated at the right hand of the Father, and he is truly present in the Eucharist. He does not leave heaven or travel from place to place. He is in heaven in his natural manner of existing and on the altar sacramentally, by the power of God.
Drafted from primary sources. Every quotation is checked against the page it links to.
14 quotations in date order
“…and lo, I am with you always, to the close of the age.”
“So then the Lord Jesus, after he had spoken to them, was taken up into heaven, and sat down at the right hand of God. And they went forth and preached everywhere, while the Lord worked with them and confirmed the message by the signs that attended it.”
“He has acknowledged the cup (which is a part of the creation) as His own blood, from which He bedews our blood; and the bread (also a part of the creation) He has established as His own body, from which He gives increase to our bodies.”
“For Elias left a sheepskin to his disciple, but the Son of God ascending left to us His own flesh! Elias indeed, cast off his mantle, before he went up; but Christ left it behind for our sakes; and yet retained it when He ascended.”
“Oh! What a marvel! What love of God to man! He who sits on high with the Father is at that hour held in the hands of all, and gives Himself to those who are willing to embrace and grasp Him.”
“And was carried in His Own Hands: how carried in His Own Hands? Because when He commended His Own Body and Blood, He took into His Hands that which the faithful know; and in a manner carried Himself, when He said, This is My Body.”
“The body which is born of the holy Virgin is in truth body united with divinity, not that the body which was received up into the heavens descends, but that the bread itself and the wine are changed into God's body and blood.”
“Now it is evident that Christ's body does not begin to be present in this sacrament by local motion. First of all, because it would follow that it would cease to be in heaven: for what is moved locally does not come anew to some place unless it quit the former one.”
“Christ's body is not in this sacrament in the same way as a body is in a place, which by its dimensions is commensurate with the place; but in a special manner which is proper to this sacrament. Hence we say that Christ's body is upon many altars, not as in different places, but "sacramentally": and thereby we do not understand that Christ is there only as in a sign, although a sacrament is a kind of sign; but that Christ's body is here after a fashion proper to this sacrament, as stated above.”
“For neither are these things mutually repugnant,-that our Saviour Himself always sitteth at the right hand of the Father in heaven, according to the natural mode of existing, and that, nevertheless, He be, in many other places, sacramentally present to us in his own substance, by a manner of existing, which, though we can scarcely express it in words, yet can we, by the understanding illuminated by faith, conceive, and we ought most firmly to believe, to be possible unto God”
“The unique and indivisible existence of the Lord glorious in heaven is not multiplied, but is rendered present by the sacrament in the many places on earth where Mass is celebrated.”
“Henceforth Christ is seated at the right hand of the Father: "By 'the Father's right hand' we understand the glory and honour of divinity, where he who exists as Son of God before all ages, indeed as God, of one being with the Father, is seated bodily after he became incarnate and his flesh was glorified."”
“"Christ Jesus, who died, yes, who was raised from the dead, who is at the right hand of God, who indeed intercedes for us," is present in many ways to his Church: in his word, in his Church's prayer, "where two or three are gathered in my name," in the poor, the sick, and the imprisoned, in the sacraments of which he is the author, in the sacrifice of the Mass, and in the person of the minister. But "he is present . . . most especially in the Eucharistic species."”