That the People of the Book may know that they have no power whatever over the Grace of Allah, that (His) Grace is (entirely) in His Hand, to bestow it on whomsoever He wills. For Allah is the Lord of Grace abounding. (29)
Tafsir
‘Believers,’ here refer to those who embraced the faith of Jesus. Those who had faith in the previous prophet and now, having discovered the Truth in the call of the Last of the Prophets, adopt his faith, are entitled to a double reward. Similarly, if those who are Muslims by birth study Islam afresh, and with renewed Islamic consciousness, become believers or Muslims, they too will be entitled to a double reward in the eyes of God.
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