the New Covenant is built on much better promises of God. One is the eternal life and the indwelling Holy Spirit in his people, being joined as one spirit with Him.
Hebrews 8:6 But in fact the ministry Jesus has received is as superior to theirs as the covenant of which he is mediator is superior to the old one, since the new covenant is established on better promises.
when Paul wrote his Epistle to the Romans, it is under the New Covenant, this is what he tells Gentile Christians:
"
13 I am talking to you Gentiles. Inasmuch as I am the apostle to the Gentiles, I take pride in my ministry
14 in the hope that I may somehow arouse my own people to envy and save some of them.
15 For if their rejection brought reconciliation to the world, what will their acceptance be but life from the dead?
16 If the part of the dough offered as firstfruits is holy, then the whole batch is holy; if the root is holy, so are the branches.
17 If some of the branches have been broken off, and you, though a wild olive shoot, have been grafted in among the others and now share in the nourishing sap from the olive root,
18 do not consider yourself to be superior to those other branches. If you do, consider this: You do not support the root, but the root supports you.
19 You will say then, “Branches were broken off so that I could be grafted in.”
20 Granted. But they were broken off because of unbelief, and you stand by faith. Do not be arrogant, but tremble.
21 For if God did not spare the natural branches, he will not spare you either.
22 Consider therefore the kindness and sternness of God: sternness to those who fell, but kindness to you, provided that you continue in his kindness. Otherwise, you also will be cut off.
23 And if they do not persist in unbelief, they will be grafted in, for God is able to graft them in again.
24 After all, if you were cut out of an olive tree that is wild by nature, and contrary to nature were grafted into a cultivated olive tree, how much more readily will these, the natural branches, be grafted into their own olive tree!
(chapter 11, verses 13-24)
Likewise in Hebrews chapter 3, also to believing Christians;
"
7 So, as the Holy Spirit says:“Today, if you hear his voice,
8 do not harden your hearts as you did in the rebellion, during the time of testing in the wilderness,
9 where your ancestors tested and tried me, though for forty years they saw what I did.
10 That is why I was angry with that generation; I said, ‘Their hearts are always going astray and they have not known my ways.’
11 So I declared on oath in my anger, ‘They shall never enter my rest.’ ”
12 See to it, brothers and sisters, that none of you has a sinful, unbelieving heart that turns away from the living God.
13 But encourage one another daily, as long as it is called “Today,” so that none of you may be hardened by sin’s deceitfulness.
14 We have come to share in Christ, if indeed we hold our original conviction firmly to the very end.
15 As has just been said: “Today, if you hear his voice, do not harden your hearts as you did in the rebellion.”
16 Who were they who heard and rebelled? Were they not all those Moses led out of Egypt?
17 And with whom was he angry for forty years? Was it not with those who sinned, whose bodies perished in the wilderness?
18 And to whom did God swear that they would never enter his rest if not to those who disobeyed?
19 So we see that they were not able to enter, because of their unbelief." (7-19)
