"Allah is your Lord and the Lord of your first forefathers." Whoever reads it in the nominative case (al-raf'), it is a new sentence. Whoever reads it in the accusative case (al-nasb), the meaning is: "And you leave the best of creators, Allah, your Lord and the Lord of your first forefathers."
﴿اللَّه ربكُم وربّ آبائكم الْأَوَّلين﴾ من قَرَأَهَا بِالرَّفْع؛ فَهُوَ كَلَام مُسْتَقْبل، وَمن قَرَأَهَا بِالنّصب؛ فَالْمَعْنى وتذرون أحسن الْخَالِقِينَ اللَّه ربكُم وَرب آبائكم الْأَوَّلين.