Abraham Believed God
As he does in Romans (Romans 4:3), Paul, quoting Genesis 15:6, uses Abraham as proof that there has never been any other way of salvation than by grace through faith. Even the Old Testament teaches justification by faith.
Wiersbe writes: Paul begins by quoting Moses to show that God’s righteousness was placed to Abraham’s account only because he believed God’s promise (Genesis 15:6). The words "accounted" in Galatians 3:6 and "counted" in Genesis 15:6 mean the same as "imputed" in Romans 4:11, 22–24. The Greek word means “to put to one’s account.” When the sinner trusts Christ, God’s righteousness is put to his account. More than this, the believer’s sins are no longer put to his account (see Romans 4:1–8). This means that the record is always clean before God, and therefore the believer can never be brought into judgment for his sins.
Galatians 3:6New American Standard Bible: 1995 Update
Even so Abraham BELIEVED GOD, AND IT WAS RECKONED TO HIM AS RIGHTEOUSNESS.