Genesis 15:6

The Holy Blessed One considered as meritorious and righteous the faith that Avram demonstrated. Another interpretation: "By what will I know?" He did not ask Him for a sign. Rather he said before Him, "Let me know by what merit they will uphold it." The Holy Blessed One replied to him: "By the merit of the sacrifices."

And he believed in the Lord, and had faith in the (Memra) Word of the Lord, and He reckoned it to him for righteousness (lizeku), because he parleyed not before him with words.

AND HE BELIEVED IN THE ETERNAL; AND HE ACCOUNTED IT TO HIM FOR RIGHTEOUSNESS. Rashi’s explanation is: The Holy One, blessed be He, accounted it to Abraham for righteousness and merit because of the faith with which he had trusted in Him.
But I do not understand the nature of this merit. Why should he not believe in the G-d of truth, and he himself is the prophet, and G-d is not a man, that He should lie? Furthermore, he who believed [and on the basis of this belief was ready] to sacrifice his only son, the beloved one, and withstood the rest of the trials, how could he not believe a good tiding?
The correct interpretation appears to me to be that the verse is stating that Abraham believed in G-d and he considered it due to the righteousness of the Holy One, blessed be He, that He would give him a child under all circumstances, and not because of Abram’s state of righteousness and his reward, even though He told him, Your reward shall be very great. Thus from now on he would no longer have to fear that sin might prevent the fulfillment of the promise. Now although in the case of the first prophecy Abraham had thought that the promise was conditional upon the recompense for his deeds, yet now since He promised him that he should have no fear on account of sin and that He will give him a child, he believed that the thing is established by G-d, truth He will not turn from it. For since this is a matter of the righteousness of G-d, it has no break in continuity, even as it is written, By Myself have I sworn, saith the Eternal, the word is gone forth from My mouth in righteousness, and shall not come back.
It may be that the verse is stating that Abraham believed that he would have a child as an heir under all circumstances, but the Holy One, blessed be He, accounted to him that this promise He had assured him would in addition be as righteousness since in His righteousness G-d did so, just as it says, G-d thought it for good. A similar case is the verse regarding Phinehas: And that was accounted unto him for righteousness, meaning that the trust he [Phinehas] has in G-d when committing that particular deed was accounted as righteousness unto all generations since G-d will forever keep His righteousness and kindness for every generation on account of [Phinehas’ deed, and this recompense transcends any strictly merited reward]. This is similar to that which is stated, Forever will I keep for him My mercy.

והאמין בה, he had full confidence that G’d would carry out all that He had promised, even though to an unprejudiced observer, the matter looked extremely unlikely and unnatural.

ויחשבה לו צדקה, G’d credited Avram with having performed an act of faith which deserved to be described as an act of righteousness. This verse was written primarily in order to prevent us from considering Avram’s query במה אדע כי אירשנה, as an expression of doubt in G’d’s promise to him. (verse 8) If Avram had retracted even in the slightest from his firm trust in G’d’s promise, what would have been the point in the Torah explicitly telling us that G’d rated his trust as so praiseworthy? Ezekiel, dealing with such fickle faith in G’d, asks ובשוב צדיק מצדקתו ויעשה עול...כל צדקותיו אשר עשה לא תזכרנה “if a righteous person were to renege from his righteousness and commit evil, none of the righteous deeds he had ever performed would be worth mentioning.” (Ezekiel 18,24)

'והאמין בה, he had already believed in G’d at the start of this prophecy, his doubts having concerned only the interpretation of the word זרעך as we have explained on verse 3. Now that G’d had added that the זרעך He had been speaking about meant biological offspring, i.e. ממעיך, and He had shown him the parable with his own eyes by showing him the stars in a prophetic vision, Avram believed in the original promise with even greater certainty, not entertaining any doubt whatsoever.

ויחשבה לו צדקה, G’d credited Avram with having performed an act of righteousness by believing so unquestioningly in His promise that he personally would become a father. In view of the fact that both he and his wife were in the process of aging, such faith was even more remarkable.