Here is what Psalm 110 says in the first four verses,

1 The LORD says to my Lord: “Sit at my right hand, until I make your enemies your footstool.”

2 The LORD sends forth from Zion your mighty scepter. Rule in the midst of your enemies!

3 Your people will offer themselves freely on the day of your power, in holy garments; from the womb of the morning, the dew of your youth will be yours.

4 The LORD has sworn and will not change his mind, “You are a priest forever after the order of Melchizedek.”

Psalm 110:1-4 - ESV

It is important to realise what great significance the New Testament attributes to the closing declaration in that passage. The closing declaration says, “You are a priest forever after the order of Melchizedek.” The Letter to the Hebrews uses this declaration as the basis of its main topic, namely, that we have a high priest who ministers in the presence of God. The Letter to the Hebrews declares - with the authority of the prophetic word of Psalm 110:4 - that we now have a high priest who is Jesus Christ the Lord. (See Hebrews 8:1 and Hebrews 4:14.)

This is wonderful news indeed! This is the news that we have a great high priest, Jesus, the Son of God! It has far reaching and marvellous consequences in our daily lives of faith as Christians here on Earth. The Son of God who came and lived among us as one of us, became for us a high priest in the service of God.

The Letter to the Hebrews gives us this initial introduction to His role as High Priest,

Therefore, in all things He (Jesus) had to be made like His brethren, that He might be a merciful and faithful High Priest in things pertaining to God, to make propitiation for the sins of the people. For in that He Himself has suffered, being tempted, He is able to aid those who are tempted.

Hebrews 2:17-18 - NKJV

That passage reveals to us that Jesus the Lord - as a merciful and faithful high priest - made propitiation for our sins and now He always lives to aid us. Propitiation, by the way, is another word for atonement which, in the New Testament context, means "at-one-ment with God." As we live in this position of “at-one-ment”, Jesus the Lord, as our high priest, is ever available to aid us whenever we find ourselves, as the Amplified Bible puts it, "being tempted and tested and tried [and who therefore are being exposed to suffering]."

The nature of this aid, given to us by Jesus our high priest, is gradually expanded on in the Letter to the Hebrews. We find within it, two precious assurances given to us regarding His aid. They are assurances given for us to lay hold of and make mention of in our prayers. Here are those two assurances:

Therefore He (Jesus) is also able to save to the uttermost those who come to God through Him, since He always lives to make intercession for them.

And having a High Priest (Jesus) over the house of God, let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience and our bodies washed with pure water.

Hebrews 7:25 - NKJV

Hebrews 10:21-22 - NKJV