وَلَا تَكُونُوا۟ كَٱلَّذِينَ نَسُوا۟ ٱللَّهَ فَأَنسَىٰهُمْ أَنفُسَهُمْ ۚ أُو۟لَـٰٓئِكَ هُمُ ٱلْفَـٰسِقُونَ Qur’an Al-Hashr (59:19)Wala takoonoo kaallatheena nasoo Allaha faansahum anfusahum olaika humu alfasiqoona
I.e., by having made a deliberately wrong use of the faculty of reason with which God has endowed man, and - by remaining oblivious of Him - having wasted their own spiritual potential.
To forget Allah is to forget the only Eternal Reality.