In the series “10 reasons why Melchizedek was actually the pre-incarnate Word of God” the following article is reason #8.
Melchizedek was without sin.
Who needeth not daily, as those high priests, to offer up sacrifice, first for his own sins, and then for the people’s: for this he did once, when he offered up himself. (Hebrews 7:27)
According to Hebrews 7:27, one of the key differences between the Levitical Priesthood and that of Mechizedek’s is that the high priests of the former needed to first offer up a sacrifice for their own sins before sacrificing on behalf of the people’s sins. In contrast, that prerequisite of sacrificing for the high priest’s personal sins is not needed in the Melchizedek priesthood. This can only mean that the high priest of the Melchizedek priesthood is already without sin. This conclusion is confirmed in Hebrews 7:28 were we read that Jesus is perpetually consecrated. Specifically, it says:
For the law maketh men high priests which have infirmity; but the word of the oath, which was since the law, maketh the Son, who is consecrated for evermore. (Hebrews 7:28)
Therefore, if being without sin is a characteristic of the Melchizedek priesthood and Jesus is a priest forever after the order and similitude of Melchizedek, then it follows that Melchizedek was also without sin. Yet, in all of history there has only been one person who is without sin, for Matthew 19:17 reminds us that there is no one who is immutably good and incapable of reproach except God. 1 Kings 8:46 says that there is no man who does not sin. 1 John 1:8 says that If we say that we have no sin, we deceive ourselves, and the truth is not in us. So we know by these verses that mere men cannot be considered sinless. The compelling conclusion that follows from all of this is that Melchizedek must be Christ the God-man.
The Christ Plus Zero Movement 