The Quran

وَلَو شاءَ اللَّهُ لَجَعَلَهُم أُمَّةً واحِدَةً وَلٰكِن يُدخِلُ مَن يَشاءُ في رَحمَتِهِ ۚ وَالظّالِمونَ ما لَهُم مِن وَلِيٍّ وَلا نَصيرٍ

Muhammad Asad

Now had God so willed, He could surely have made them all one single community:¹ none the less, He admits unto His grace him that wills [to be admitted]² whereas the evildoers shall have none to protect them and none to succour them [on Judgment Day].

The implication being, "but He has not willed it": see second paragraph of 5:48 and the corresponding notes 66 and 67; 16:93 and note 116; also note 29 on 10:19.
Or: "He admits whomever He wills unto His grace" - similar to the double meaning inherent in the oft-recurring phrase, Allahu yahdi man yasha'u wa-yudillu man yasha'u, which can be understood either as "God guides whomever He wills and lets go astray whomever He wills", or, alternatively, as "God guides him that wills [to be guided] and let go astray him that wills [to go astray]". See, in particular, Zamakhshari's elaborate comment on this problem quoted in note 4 on the second half of 14:4.