Question:

The Old Covenant royal law of love is that upon which Christ said all the Law and Prophets hang. What do you think is the essential difference to how that was kept or fulfilled under the Old Covenant of Law and how it is kept or fulfilled under the New Covenant of Grace?

Answer:

Paul, speaking of those who tried to fulfill the law under the Old Covenant, said: "For they being ignorant of God's righteousness, and going about to establish their own righteousness, have not submitted themselves to the righteousness of God." Romans 10:3 It is not that it wasn't possible for them to fulfill by looking forward in faith to what Christ accomplished just as we look back in faith to what Christ accomplished. Whether then or in the present time, we access the Grace of God which is through faith in Christ in God's eternal Now. It is not that it wasn't possible, but it was that they were ignorant of it. However, some touched it back then and entered into it to some degree, even as some Christians today also enter into it to some degree. But, even today, some are ignorant of God's righteousness and go about to establish their own righteousness, not submitting to the righteousness of God.

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We must remember that the Royal Law is Love and Love fulfills all the commandments. And we must remember that God is Love. Righteousness is simply God working His work through us. All of this is only available through Christ. We were designed by God and created to have Him work His Righteousness through us in thought, word, and deed, every moment of every day.

It starts with God. He leads us. This is how the faith of God comes to us. God speaks. "Faith cometh by hearing and hearing by [the] word (literally, utterance) of God." Romans 10:17 Note that "the" is not really in this sentence in the original text.

(some caveats about hearing God's Voice)

Then, faith gives us access to grace. Grace is literally the free gift of God. "By whom (Jesus) also we have access by faith into this grace wherein we stand" Romans 5:2

We then have opportunity to yield to the Grace of God and let the Grace of God do the work in and through us. "But by the grace of God I am what I am: and his grace which was bestowed upon me was not in vain; but I laboured more abundantly than they all: yet not I, but the grace of God which was with me." 1 Corinthians 15:10

That Grace is the free gift of righteousness. Righteousness is the out working of the love of God. His Righteousness/Love flows out through us as a Free Gift from Him. "The love of God is shed abroad in our hearts by the Holy Ghost." Romans 5:5 "He that believeth on me, as the scripture hath said, out of his belly (innermost being) shall flow rivers of living water. (But this spake he of the Spirit, which they that believe on him should receive" John 7:38-39 These two verses are saying the same thing in different ways.

Our part is to yield ourselves to this free gift. We are to receive it. "Likewise reckon ye also yourselves to be dead indeed to sin, but alive to God through Jesus Christ our Lord. Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof. Neither yield ye your members as instruments of unrighteousness to sin: but yield yourselves to God, as those that are alive from the dead, and your members as instruments of righteousness to God. For sin shall not have dominion over you: for ye are not under the law, but under grace." Romans 6:11-14

You can read more about this here.