﴿ ثُمَّ تَوَلَّيْتُم مِّنۢ بَعْدِ ذَٰلِكَ ۖ فَلَوْلَا فَضْلُ ٱللَّهِ عَلَيْكُمْ وَرَحْمَتُهُۥ لَكُنتُم مِّنَ ٱلْخَٰسِرِينَ ﴾

“And you turned away after that-! And had it not been for God's favour upon you and His grace, you would surely have found yourselves among the lost;”

(Then, even after that, you turned away) you turned your back on the covenant, (and if it had not been for the grace of Allah upon you) by deferring His punishment (and His mercy) by sending Muhammad (pbuh) to you (you had been among the losers) you would have among the duped who are exposed to Allah's punishment.