Hebrews 8:3-5
2 In the next place that you may understand that he used the word “minister” of the manhood, observe how he again indicates it: “For” (he says) “every high priest is ordained to offer both gifts and sacrifices, wherefore it is of necessity that this man have somewhat also to offer.”
Do not now, because you hear that He sits, suppose that His being called High Priest is mere idle talk. For the former, viz. His sitting, belongs to the dignity of the Godhead, but this to His great lovingkindness, and His tender care for us. On this account he repeatedly urges this very thing, and dwells more upon it: for he feared lest the other [truth] should overthrow it. Therefore he again brings down his discourse to this: since some were enquiring why He died. He was a Priest. But there is no Priest without a sacrifice. It is necessary then that He also should have a sacrifice.
And in another way; Having said that He is on high, he affirms and proves that He is a Priest from every consideration, from Melchisedec, from the oath, from offering sacrifice. From this he also frames another and necessary syllogism. “For if” (he says) “He had been on earth, He would not be a Priest, seeing that there are priests who offer the gifts according to the Law.” If then He is a Priest (as He really is), we must seek some other place for Him. “For if He were” indeed “on earth, He should not be a priest.” For how [could He be]? He offered no sacrifice, He ministered not in the Priest's office. And with good reason, for there were the priests. Moreover he shows, that it was impossible that [He] should be a priest upon earth. For how [could He be]? There was no rising up against [the appointed Priests], he means.
3. Here we must apply our minds attentively, and consider the Apostolic wisdom; for again he shows the difference of the Priesthood. “Who” (he says) “serve unto the example and shadow of heavenly things.”
What are the heavenly things he speaks of here? The spiritual things. For although they are done on earth, yet nevertheless they are worthy of the Heavens. For when our Lord Jesus Christ lies slain [as a sacrifice], when the Spirit is with us, when He who sits on the right hand of the Father is here, when sons are made by the Washing, when they are fellow citizens of those in Heaven, when we have a country, and a city, and citizenship there, when we are strangers to things here, how can all these be other than “heavenly things”? But what! Are not our Hymns heavenly? Do not we also who are below utter in concert with them the same things which the divine choirs of bodiless powers sing above? Is not the altar also heavenly? How? It has nothing carnal, all spiritual things become the offerings. The sacrifice does not disperse into ashes, or into smoke, or into steamy savor, it makes the things placed there bright and splendid. How again can the rites which we celebrate be other than heavenly? For when He says, “Whose soever sins ye retain they are retained, whose soever sins ye remit, they are remitted” when they have the keys of heaven, how can all be other than heavenly?
“Who” (he says) “serve unto the example and shadow of heavenly things, as Moses was admonished of God, when he was about to make the tabernacle, for see, says He, that you make all things according to the pattern showed to you in the mount.” Inasmuch as our hearing is less ready of apprehension than our sight (for the things which we hear we do not in such wise lay up in our soul, as those which we see with our very eyes), He showed him all. Either then he means this by “the example and shadow,” or else he [speaks] of the Temple. For, he went on to say, “See” (His words are), that “thou make all things according to the pattern showed to you in the mount.” Was it then only what concerned the furniture of the temple that he saw, or was it also what related to the sacrifices, and all the rest? Nay, one would not be wrong in saying even this; for The Church is heavenly, and is nothing else than Heaven.
Source: Homilies on the Epistle to the Hebrews (New Advent)