The Quran

وَما كانَ لَهُ عَلَيهِم مِن سُلطانٍ إِلّا لِنَعلَمَ مَن يُؤمِنُ بِالآخِرَةِ مِمَّن هُوَ مِنها في شَكٍّ ۗ وَرَبُّكَ عَلىٰ كُلِّ شَيءٍ حَفيظٌ

Muhammad Asad

And yet, he had no power at all over them:¹ [for if We allow him to tempt man,] it is only to the end that We might make a clear distinction between those who [truly] believe in the life to come and those who are in doubt thereof:² for thy Sustainer watches over all things.

Cf. a similar phrase placed in the mouth of Iblis in 14:22 ("I had no power at all over you: I but called you - and you responded unto me"), and the corresponding note 31; also, see note 30 on 15:39-40. - Although, on the face of it, verses 20-21 of the present surah refer to the people of Sheba, their import is (as the sequence shows) much wider, applying to the human race as such.
See 15:41 and the corresponding note 31.