Psalm 103

In modern day the Lord said -

"BEHOLD, I say unto you that all old covenants have I caused to be done away in this thing; and this is a new and an everlasting covenant, even that which was from the beginning." (Doctrine and Covenants | Section 22:1)

I can't help but note that this is an everlasting covenant which was known from the beginning. David taught in psalm the essential nature of this covenant -

"But the mercy of the LORD is from everlasting to everlasting upon them that fear him, and his righteousness unto children's children;
18 To such as keep his covenant, and to those that remember his commandments to do them." (Old Testament | Psalms 103:17 - 18)

What is everlasting within this covenant is a everlasting promise of mercy to those who keep this covenant and remember and obey His commandments. I also note the promise of extended righteousness to the faithful's posterity. Though the new and everlasting covenant includes ordinances and greater blessings, again the key to the entire plan of salvation is everlasting mercy.