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

(Saheeh International)

And of the people is he who worships Allāh on an edge.¹ If he is touched by good, he is reassured by it; but if he is struck by trial, he turns on his face [to the other direction]. He has lost [this] world and the Hereafter. That is what is the manifest loss.

At the edge of his religion, so to speak, i.e., with uncertainty, hypocrisy or heedlessness.