19 And this greater priesthood administereth the gospel and holdeth the key of the mysteries of the kingdom, even the key of the knowledge of God.
20 Therefore, in the ordinances thereof, the power of godliness is manifest.
Why “therefore”? I think it’s saying that the Melchizedek Priesthood has the keys of the mysteries and of the knowledge of God, therefore, when you receive an ordinance of the gospel through the Melchizedek priesthood, that ordinance contains something of those mysteries and knowledge? Or, they have the key that open those doors, and they way they help others through those doors are by the ordinances?
21 And without the ordinances thereof, and the authority of the priesthood, the power of godliness is not manifest unto men in the flesh;
I was trying to figure out how the word “godliness” was being used here, and I finally thought to do a search. I found this scripture in 2 Timothy and I’m almost positive D&C 84 is referring to it: “2 Timothy 3:5 “Having a form of godliness, but denying the power thereof: from such turn away.”) I think that’s got to be the reference! I’m excited to keep thinking that through.
22 For without this no man can see the face of God, even the Father, and live.
23 Now this Moses plainly taught to the children of Israel in the wilderness, and sought diligently to sanctify his people that they might behold the face of God;
24 But they hardened their hearts and could not endure his presence; therefore, the Lord in his wrath, for his anger was kindled against them, swore that they should not enter into his rest while in the wilderness, which rest is the fulness of his glory.
Search for “godliness” and “power” had just a few references: https://www.lds.org/scriptures/search?lang=eng&query=godliness+power&x=0&y=0