بَلْ قُلُوبُهُمْ فِي غَمْرَةٍ (But their hearts are in ignorance) Lexically غَمْرَةٍ means deep water where a person may drown or be overwhelmed. For this reason the word is also used to signify a covering or something that conceals an object. In this verse the word means the ignorance in which the hearts of the infidels are submerged and hidden so that no light can reach them.
وَلَهُمْ أَعْمَالٌ مِّن دُونِ ذَٰلِكَ
And they have deeds, other than that - 23:63
It means that in addition to their rejection of the Oneness of Allah, and their denial of the true faith, they engage themselves in other sinful deeds.