Heb 6:13-14,17, Gal 3:16-17, Jer 34:18-20
In the Old Testament, two people who made a promise cut an animal and passed between them. This means that anyone who does not keep this promise will be split apart and die like this animal. (Jeremiah 34:18-20)
Jeremiah 34:18 And I will give the men that have transgressed my covenant, which have not performed the words of the covenant which they had made before me, when they cut the calf in twain, and passed between the parts thereof, 19 The princes of Judah, and the princes of Jerusalem, the eunuchs, and the priests, and all the people of the land, which passed between the parts of the calf; 20 I will even give them into the hand of their enemies, and into the hand of them that seek their life: and their dead bodies shall be for meat unto the fowls of the heaven, and to the beasts of the earth.
God has promised us blessings and walked alone through the pieces of meat. This means that God will surely keep this promise. (Gen 15:17)
Gen 15:17 And it came to pass, that, when the sun went down, and it was dark, behold a smoking furnace, and a burning lamp that passed between those pieces.
God has promised to bless us by swearing by Himself. (Heb 6:13-14,17)
Heb 6:13 For when God made promise to Abraham, because he could swear by no greater, he sware by himself, 14 Saying, Surely blessing I will bless thee, and multiplying I will multiply thee. 17 Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath:
The unchanging covenant that God has sworn is Christ. He has blessed us through Christ. (Gal 3:16-17)
Gal 3:16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ. 17 And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect.