Genesis 15:6 Then he believed in the LORD; and He reckoned it to him as righteousness.

Gen 15:6  Then he believed in the LORD; and He reckoned it to him as righteousness.

וְהֶאֱמִן בַּיהוָה וַיַּחְשְׁבֶהָ לּוֹ צְדָקָֽה                    

In whom does Abram believe?
the LORD

What is reckoned to him?
it

How is it reckoned to him?
“. . . as righteousness”  (Gen 15:6).

Was Abraham justified by his faith in God before or after the Law of circumcision?
Before

How does Romans 4 explain “Abraham believed God, and it was credited to him as righteousness”  (Rom 4:3)?
Rom. 4:1   What then shall we say that Abraham,  our forefather  according to the flesh, has found?  2 For if Abraham was justified  by works, he has something to boast about, but not before God.  3 For what does the Scripture say? “ABRAHAM BELIEVED GOD, AND IT WAS CREDITED TO HIM AS RIGHTEOUSNESS.”  4 Now to the one who  works, his wage is not credited as a favor, but as what is due.  5 But to the one who does not work, but  believes in Him who justifies the ungodly, his faith is credited as righteousness,  6 just as David also speaks of the blessing on the man to whom God credits righteousness apart from works:
7      “ BLESSED ARE THOSE WHOSE LAWLESS DEEDS HAVE BEEN FORGIVEN,
    AND WHOSE SINS HAVE BEEN COVERED.
8      “ BLESSED IS THE MAN WHOSE SIN THE LORD WILL NOT  TAKE INTO ACCOUNT.” 
Rom. 4:9   Is this blessing then on   the circumcised, or on  the uncircumcised also? For  we say, “FAITH WAS CREDITED TO ABRAHAM AS RIGHTEOUSNESS.”  10 How then was it credited? While he was  circumcised, or  uncircumcised? Not while  circumcised, but while  uncircumcised;  11 and he  received the sign of circumcision,  a seal of the righteousness of the faith which  he had while uncircumcised, so that he might be  the father of  all who believe without being circumcised, that righteousness might be credited to them,  12 and the father of circumcision to those who not only are of the circumcision, but who also follow in the steps of the faith of our father Abraham which  he had while uncircumcised.
Rom. 4:13   For  the promise to Abraham or to his  descendants  that he would be heir of the world was not  through the Law, but through the righteousness of faith.  14 For  if those who are  of the Law are heirs, faith is made void and the promise is nullified;  15 for  the Law brings about wrath, but  where there is no law, there also is no violation.
Rom. 4:16   For this reason it is  by faith, in order that it may be in accordance with  grace, so that the promise will be guaranteed to  all the  descendants, not only to  those who are of the Law, but also to   those who are of the faith of Abraham, who is  the father of us all,  17 (as it is written, “A FATHER OF MANY NATIONS HAVE I MADE YOU”) in the presence of Him whom he believed, even God,  who gives life to the dead and   calls into being  that which does not exist.  18 In hope against hope he believed, so that he might become  a father of many nations according to that which had been spoken, “ SO SHALL YOUR  DESCENDANTS BE.”  19 Without becoming weak in faith he contemplated his own body, now  as good as dead since  he was about a hundred years old, and  the deadness of Sarah’s womb;  20 yet, with respect to the promise of God, he did not waver in unbelief but grew strong in faith,  giving glory to God,  21 and  being fully assured that  what God had promised, He was able also to perform.  22 Therefore  IT WAS ALSO CREDITED TO HIM AS RIGHTEOUSNESS.  23 Now  not for his sake only was it written that it was credited to him,  24 but for our sake also, to whom it will be credited, as those  who believe in Him who  raised Jesus our Lord from the dead,  25 He who was  delivered over because of our transgressions, and was  raised because of our justification.

Adonai,
You are just and the One Who justifies. Glory and thanks to You through Jesus for Your free gift of eternal life in Christ Jesus our Lord. For all have sinned and fall short of Your glory, being justified as a gift by Your grace through the redemption which is in Christ Jesus; whom You displayed publicly as a propitiation in His blood through faith. This was to demonstrate Your righteousness, because in Your forbearance You passed over the sins previously committed; for the demonstration of Your righteousness at the present time, so that You would be just and the justifier of the one who has faith in Jesus. You are the God of the Jews and the God of the Gentiles. Glory and praise to Your great Name. And glory to Jesus the Messiah Who came and is coming again. In the Name of Jesus I pray, Amen.