“For the law appoints as high priests men who are weak; but the oath, which came after the law, appointed the Son, who has been made perfect forever” ( Hebrews 7:28 )
The entire book of Hebrews contrasts in one way or the other the vast differences between the Old Covenant and the New. Whether it is contrasting Jesus’ priesthood with the Levitical priesthood or the outcome of those priestly duties, the result is the same, the New Covenant is far superior in every way imaginable ( Hebrews 8:6 ).
The point of the entire book is to show us that the Covenant of the Law is inferior to the Covenant which Jesus ushered in and secures for us. In fact the writer goes to great lengths to describe that this Covenant is backed by an oath from God that it is eternal and permanent. Because of the oath, God will never revoke or change his mind concerning these promises ( Hebrews 6:16-18, Hebrews 7:18-22 ).
This particular verse is a prime example of this contrast. The law sets up men to intercede based on frailty and weakness, which is not very effective considering the job they are required to do. However, the New Covenant, based on the oath that Jesus is the one to make peace for us ( Romans 5:1 ), is very effective because Jesus is already perfect and sinless. In other words, he fulfilled the requirements of approaching God in perfection, so He is able to perfectly intercede on our behalf ( I Timothy 2:5 ). Not only is this so, but since He defeated death once for all His position is eternal since death no longer has claim over Him ( Romans 4:25, Romans 6:9 ). This give us both confidence and assurance of every promise made to us throughout the Scriptures because Jesus is the guarantee of everything written to us ( Hebrews 7:22 ).
When you really break it down, the Law is what strengthens sin in our lives showing our guilt and shame ( Romans 3:19-20, Galatians 3:10-11, Galatians 3:21 ). Then when you consider that those set up to make intercession are subject to the same weaknesses, we can see the need for a new system. “The former regulation is set aside because it was weak and useless (for the law made nothing perfect), and a better hope is introduced, by which we draw near to God” ( Hebrews 7:18-19 ).
The better hope is Jesus. He is the only one that can make us righteous and acceptable to God ( Romans 3:22, 4:5, 16, Romans 5:1, II Corinthians 5:21, Titus 3:5-7 ). He took our sin and gave us His righteousness ( II Corinthians 5:21 ) so that we can stand justified before God ( Romans 8:30 ). Since we now can stand in His presence we also know that we have boldness in approaching Him by faith ( Hebrews 4:16, Hebrews 10:22 ). This is by far a more superior and advantageous covenant in every way imaginable.
This Covenant is backed by an oath, not the law, “Because of this oath, Jesus has become the guarantee of a better covenant” ( Hebrews 7:22 ). And the oath is founded on better promises, “But the ministry Jesus has received is as superior to theirs as the covenant of which he is mediator is superior to the old one, and it is founded on better promises” ( Hebrews 8:6 ). This is why the contrast between the law and the oath is so great and is very important to us to understand because we are heirs of the New Covenant, founded and secured in Jesus Christ. “For the Law sets up men in their weakness [frail, sinful, dying human beings] as high priests, but the word of [God's] oath, which [was spoken later] after the institution of the Law, [chooses and appoints as priest One Whose appointment is complete and permanent], a Son Who has been made perfect forever” ( Hebrews 7:28 AMP).