The Quran

وَمِنَ النّاسِ مَن يَعبُدُ اللَّهَ عَلىٰ حَرفٍ ۖ فَإِن أَصابَهُ خَيرٌ اطمَأَنَّ بِهِ ۖ وَإِن أَصابَتهُ فِتنَةٌ انقَلَبَ عَلىٰ وَجهِهِ خَسِرَ الدُّنيا وَالآخِرَةَ ۚ ذٰلِكَ هُوَ الخُسرانُ المُبينُ

Muhammad Asad

And there is, too, among men many a one who worships God on the border-line [of faith]:¹ thus, if good befalls him, he is satisfied with Him; but if a trial assails him, he turns away utterly,² losing [thereby both] this world and the life to come: [and] this, indeed, is a loss beyond compare.³

I.e., wavering between belief and disbelief, and not really committed to either.
Lit., "he turns about on his face" the "face" (wajh) of man signifying metonymically his whole being.
Lit., "the [most] obvious loss".