Christ as Melchisedec of The Old Testament... the only way I know to cover it, because the 'key' verse requires much of the chapter as preparation.
Heb 7:1-17
1 For this Melchisedec, king of Salem, priest of the most high God, who met Abraham returning from the slaughter of the kings, and blessed him;
2 To whom also Abraham gave a tenth part of all; first being by interpretation King of righteousness, and after that also King of Salem, which is, King of peace;
3 Without father, without mother, without descent, having neither beginning of days, nor end of life; but made like unto the Son of God; abideth a priest continually.
Melchisedec in Gen.14 met Abraham, blessed him, and gave him "bread and wine", symbols that would later become sacraments of The New Covenant. The Lord also met Abraham at his tent door in Gen.18 later, along with two men (angels) I believe were sent to save Lot out of Sodom and Gomorrah. The name Melchisedec means "King of righteousness".
There's only One Who has no beginning of days, nor end of life, and made like unto the Son of God. It's The Son of God Himself, our Lord Jesus Christ. That should be our first clue. The idea of abideth a priest continually is in the everlasting sense also, without end. Such is our Lord Jesus Christ.
4 Now consider how great this man was, unto whom even the patriarch Abraham gave the tenth of the spoils.
5 And verily they that are of the sons of Levi, who receive the office of the priesthood, have a commandment to take tithes of the people according to the law, that is, of their brethren, though they come out of the loins of Abraham:
The Levite priests took tithes of the people of Israel per God's law. Yet Abraham tithed to Melchisedec.
6 But he whose descent is not counted from them received tithes of Abraham, and blessed him that had the promises.
7 And without all contradiction the less is blessed of the better.
8 And here men that die receive tithes; but there he receiveth them, of whom it is witnessed that he liveth.
Melchisedec didn't descend from the Levites though. They came later, as Levi was still unborn in the loins of Abraham. The lesser is blessed by the greater, so how could Melchisedec bless Abraham, since even Levi wasn't yet born? Not only that, God's law wasn't given yet either when Melchisedec met Abraham, and Abraham tithed to him.
9 And as I may so say, Levi also, who receiveth tithes, payed tithes in Abraham.
10 For he was yet in the loins of his father, when Melchisedec met him.
11 If therefore perfection were by the Levitical priesthood, (for under it the people received the law,) what further need was there that another priest should rise after the order of Melchisedec, and not be called after the order of Aaron?
Even Levi gave tithes, having come from the seed of Abraham. But Levi wasn't born yet when Melchisedec met Abraham. And if perfection was by the Levitical priesthood, then why would another need be born after Melchisedec's order, instead of from Aaron?
12 For the priesthood being changed, there is made of necessity a change also of the law.
13 For he of whom these things are spoken pertaineth to another tribe, of which no man gave attendance at the altar.
14 For it is evident that our Lord sprang out of Juda; of which tribe Moses spake nothing concerning priesthood.
There's one of the 'keys'. The "For he of whom these things are spoken pertaineth to another tribe" points back to the very start of this chapter, about the one who has been spoken about so far. And who has that been up until these verses? Melchisedec. But right here Paul links the identity of Melchisedec with One Who was born of Israel, of the tribe of Judah.
That's another clue where this Heb.7 chapter is pointing directly to Christ as the Person of Melchisedec Who met Abrahaim in Gen.14.
15 And it is yet far more evident: for that after the similitude of Melchisedec there ariseth another priest,
After the "similtude of Melchisedec"? This is talking about Christ being after the similtude of Melchisedec. It's direct connection to Christ. And just what KIND of similtude is that?...
16 Who is made, not after the law of a carnal commandment, but after the power of an endless life.
Not after the law of a carnal commandment, like the Levitical priesthood, but after the power of an endless life, i.e., an everlasting priesthood, with beginning, with out end. Who could that only point to? Christ Jesus, for He is our High Priest.
17 For He testifieth, Thou art a priest for ever after the order of Melchisedec.
Our Heavenly Father said that about our Lord Jesus Christ. And the idea of "for ever" don't just mean to count forward in time. It means everlasting, always existing, no beginning, no end. That's not a power attributed to just any man, but to God only.
18 For there is verily a disannulling of the commandment going before for the weakness and unprofitableness thereof.
19 For the law made nothing perfect, but the bringing in of a better hope did; by the which we draw nigh unto God.
20 And inasmuch as not without an oath He was made priest:
21 (For those priests were made without an oath; but this with an oath by Him That said unto Him, "The Lord sware and will not repent, Thou art a priest for ever after the order of Melchisedec:")
What the flesh Levitical priesthood couldn't establish because of weakness, the order of Melchisedec could. How's that? Because the order of Melchisedec is pointing to an everlasting eternal order, God's Heavenly order, without beginning without end. This is why the order of Melchisedec is applied to our Lord Jesus Christ, and can ONLY be about Christ and no other.
22 By so much was Jesus made a surety of a better testament.
23 And they truly were many priests, because they were not suffered to continue by reason of death:
24 But this man, because He continueth ever, hath an unchangeable priesthood.
Unchangeable means everlasting also, without beginning without end, and idea only applicable to God. Priests per the order of Aaron lose their priest office when they die. But this Priest never lost His Office as Priest, which is the meaning of, 'for ever after the order of Melchisedec.'
25 Wherefore He is able also to save them to the uttermost that come unto God by Him, seeing He ever liveth to make intercession for them.
26 For such an High Priest became us, Who is holy, harmless, undefiled, separate from sinners, and made higher than the heavens;
(KJV)
And once again, with the idea of "seeing He ever liveth" is another reference to Christ, even though He was born in the flesh like us, but was Perfect and without sin.
That Hebrews 7 chapter is pointing to Melchisedec of Gen.14 as Christ back in the Old Testament times, before He was born in the flesh to die on the cross. It makes sense then, why Melchisedec would offer Abraham bread and wine, seeing how Apostle Paul taught that Abraham's faith, 430 years before the law, represents the Faith on Jesus Christ that all Christians have believed (Gal.3).
So Melchisedec in the OT is a pointer to Christ as our High Priest PRIOR to the Old Covenant, and PRIOR to the New Covenant, representing His Salvation plan from the foundations of the world.