After these things the word of the LORD came unto Abram in a vision, saying, Fear not, Abram: I am thy shield, and thy exceeding great reward. Genesis 15:1

At this time Abraham has already gone on a long journey through trials. And God reminded Abraham that he is his protection and he will be blessed. But notice Abrahams response in the next verse

And Abram said, Lord GOD, what wilt thou give me, seeing I go childless, and the steward of my house is this Eliezer of Damascus? Genesis 15:2

Abraham is like “God I hear what you are saying but I still don’t have my promised Son” Abraham is about 100 years old and at this point is almost giving up on ever being a Father. Abraham decided that a servant of his household would become his heir, because Abraham gave up on Gods promise. However look how God responds

And, behold, the word of the LORD came unto him, saying, This shall not be thine heir; but he that shall come forth out of thine own bowels shall be thine heir. Genesis 15:4

God continues to remind Abraham of his promise, that he would have the promised Son from his own flesh and blood. At this point Abraham thinks this is impossible, but we know all things are possible with God

And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be. Genesis 15:5

Not only will Abraham have his promised Son, but he will be the Father of many nations, he will have descendants as numerous as the stars in the sky

And he believed in the LORD; and he counted it to him for righteousness. Genesis 15:6

From a human perspective, this all seemed impossible. But Abraham now fully believed God and was credited to him as righteousness. Notice Abraham was righteous because he had true faith in God, not by his works

Even if it seems impossible from your perspective, nothing is impossible with God. If God made a promise he will fulfill it because he can not lie!