Hebrews 5

1 A high priest devoted his entire life to learning about God and teaching others. The sacrifices described in the Mosaic Law required special attention to detail. Most people did not have the desire or aptitude to do this full time. Thus, God appointed particular men to do this.
2 We would agree that a human high priest could sympathize with our weaknesses because we know he would have faults as well. No one is infallible.
3 The evidence that the high priest was affected by sin like the rest of us was that he too had to offer sacrifices for his sins.
4 Aaron and his descendants were appointed by God to fulfill the role of high priest until the advent of Jesus (Num 16:40).
5 In the Old Testament, several men tried to take Aaron's rightful place as high priest and suffered the consequences. However, God specially appointed Jesus to replace the high priesthood of Aaron's family. While men could serve as high priest, only Jesus was perfectly qualified for the position.
6 The quote is from Psa 110:4 and refers to the encounter of Abraham and Melchizedek in Gen 14:18-20. This is given as evidence that Aaron's priesthood was not exclusive to his family, but was for whoever was appointed by God for the position.