Hebrews 8:4

For if he were on earth, he should not be a priest
The Socinians from hence attempt to prove that Christ was not a priest, and did not offer sacrifice on earth; whereas his coming into the world, and his appearance in human nature, was in the character of a priest, and to qualify himself for one; his death was his sacrifice, which was on earth; and he never offered but one sacrifice; and it was after he had offered himself that he went to heaven; so the sacrifices under the law were first offered, before their blood was carried within: but the meaning is, either if "that" was on earth, namely, what it was necessary he should have to offer; if his human nature had been earthly, had been of men, had come by ordinary generation, he had not been properly, only typically a priest, at most; and had been no better than the typical ones; yea, he would have been needless, nay, might not have offered, not being of Levi's tribe, and could not have existed as a priest with the sons of Aaron; but he had his human nature in another way, through the power of the Holy Ghost from above, and therefore is said to come from above, from heaven, and to be the Lord from heaven: or the sense is, if he was on earth, and had not died, he had not been a priest; and if he had died and remained under the power of death, he had been a priest of no account and use; and had he rose again and remained on earth, without going to heaven, with his blood and sacrifice, he had not been a perfect priest; if Christ had remained on earth, the Levitical priesthood had remained, and so he would have been no priest, since two priesthoods could not have subsisted together. The Levitical priesthood was in force while Christ was on earth; Christ's priesthood was not perfected on earth; the Levitical priesthood remaining while he was on earth, proves he was not then a perfect priest, or had not completed his priesthood; had he been so, that would not have subsisted; it was necessary therefore that Christ should enter into the holy place, to put an end to the Levitical priesthood: moreover, if he had remained on earth, he had been needless;

seeing that there are priests that offer gifts according to the law;
there were priests when Christ was upon earth; their work was to offer gifts the people brought, and sacrifices for sin, and that according to the law of Moses, which till the death of Christ was in full force.