﴿ إِلَٰهُكُمْ إِلَٰهٌۭ وَٰحِدٌۭ ۚ فَٱلَّذِينَ لَا يُؤْمِنُونَ بِٱلْءَاخِرَةِ قُلُوبُهُم مُّنكِرَةٌۭ وَهُم مُّسْتَكْبِرُونَ ﴾

“Your God is the One God: but because of their false pride, the hearts of those who do not believe in the life to come refuse to admit this [truth].”

Your God the One deserving of your worship is One God without any equal in terms of His essence or His attributes and this is God exalted He be. But as for those who do not believe in the Hereafter their hearts are in denial rejecting the Oneness of God and they are arrogant scorning to believe in it.