Romans 4:3 For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.
Galatians 3:6 Even as Abraham believed God, and it was accounted to him for righteousness.
James 2:23 And the scripture was fulfilled which saith, Abraham believed God, and it was imputed unto him for righteousness: and he was called the Friend of God.
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wel he didn't follow any religion, the judaism appears with the law of moses and the reforms during ezra, christianity appears later jesus died and islam appears with muhammad as a prophet for arabs