2 Abram said, “O Lord God, what will You give me, since I am childless, and the heir of my house is Eliezer of Damascus?” 3 And Abram said, “Since You have given no offspring to me, one born in my house is my heir.” 4 Then behold, the word of the Lord came to him, saying, “This man will not be your heir; but one who will come forth from your own body, he shall be your heir.” And He took him outside and said, “Now look toward the heavens, and count the stars, if you are able to count them.” And He said to him, “So shall your descendants be.” (Gen 15:2-5)By now Abram had already talked to God on several other occasions. He had witnessed God make a unilateral covenant. At this very point in Abram's life, would you say that Abram believed in God? Surely the answer is yes in that Abram, at this point, believed God existed. He had first-hand contact of which we can't help but be envious. He would have, we can be certain, given his full intellectual assent to a question about the reality of God.
Then we move on. The very next verse tells us:
6 Then he believed in the Lord; and He reckoned it to him as righteousness.V6 expresses a change that occurs at that point--then he believed. That belief cannot mean "he accepted that God was god and that he existed." Abram already did that. It has to mean something else. The word translated in v6 as "believed" could also be translated as "had faith" or "trusted."
I'm convinced there is no adequate word in English. Trust may be the closest. Faith is more common. This inadequacy is part of what makes the doctrine of Justification by Faith Alone so difficult to understand.
What is faith? I can only begin to grasp it by what it isn't. It is not works. Abraham was not justified because he trusted God and offered Isaac. Cause and effect are backwards in that view. No, Abraham trusted God and offered Isaac because he was justified, and that justification manifested itself as faith. Abraham was already justified before he offered Isaac. Before he did anything meritorious.
If you are saved you are justified by faith. What is that? Mentally delete everything good (in human terms) you did prior to being saved. Those are but filthy rags with no merit. Also delete everything good you did after to being saved. They may indeed be meritorious--but mentally delete them just the same. Without any of these, you are still justified by faith. Whatever is left in you after you have deleted all those good works--that is faith. And it comes from God.
The gospel in four words comes from Romans 4:5; God justifies the wicked.