يَا أَيُّهَا الَّذِينَ آمَنُوا اتَّقُوا اللَّـهَ وَآمِنُوا بِرَسُولِهِ يُؤْتِكُمْ كِفْلَيْنِ مِن رَّحْمَتِهِ (0 those who believe, fear Allah, and believe in His Messenger, and He will give you two shares [ of reward ] out of His mercy,..28) The vocative expression يَا أَيُّهَا الَّذِينَ آمَنُوا "O those who believe..." in this verse Al-Hadid 57:28 ⧉ refers to the People of the Book who believed in Prophet ` Isa (علیہ السلام) . As a general rule, the Qur'an uses this vocative expression for Muslims only, not for Jews or Christians. The expression used for them is ahlul-kitab [ People of the Book ] because their believing in Prophets Musa and ` Isa (علیہما السلام) is not tantamount to being 'believers'. They need to believe in the Holy Prophet Muliammad ﷺ . Only in that case can they be called الَّذِينَ آمَنُوا "Othose who believe". But here the Qur'an deviates from the general rule, and addresses them by the words, "Othose who believe" presumably because the concluding verse reminds the Christians that believing in Prophet ` Isa (علیہ السلام) is not sufficient: the true meaning of believing in Prophet ` Isa (علیہ السلام) dictates that they should believe in the Last Prophet ﷺ . If they do, then they deserve to be called as الَّذِينَ آمَنُوا 'those who believe'.
Then the verse promises the People of the Book that believe in the Holy Prophet ﷺ ، their reward will be doubled. The first reward is on account of their believing in Prophet ` Isa (علیہ السلام) or Musa (علیہ السلام) and following their Shari’ ah, and the second reward is for their believing in the Holy Prophet ﷺ and acting upon his Shari’ ah. The reason for their double reward is that Jews and Christians were unbelievers until they believed in the Holy Prophet ﷺ ، and no worship of an unbeliever is acceptable. It required that any action they had performed according to a previous Shari` ah would go waste. But the present verse Al-Hadid 57:28 ⧉ clarifies that when an unbeliever embraces Islam, all his good actions are restored, hence he is doubly rewarded.