"And if so, We would have given them from Us" meaning: from Our presence, "a great reward" meaning: Paradise.
﴿وَإِذا لآتيناهم من لدنا﴾ ﴿من عندنَا﴾ (أجرا عَظِيما﴾ يَعْنِي: الْجنَّة.