His saying: {And be not like those who forgot Allah}, meaning: they abandoned the remembrance of Allah with sincerity from their hearts; {so He made them forget themselves}, He abandoned them from remembering Him with sincerity. He said: {Those are the defiantly disobedient}, and it is the disobedience of polytheism (shirk).
قَوْله: ﴿وَلا تَكُونُوا كَالَّذِينَ نَسُوا الله﴾ يَعْنِي: تركُوا ذكْرَ اللَّه بالإخلاص من قُلُوبهم ﴿فأنساهم أنفسهم﴾ تَركهم من أَن يذكروها بالإخلاص لَهُ قَالَ: ﴿أُولَئِكَ هُمُ الْفَاسِقُونَ﴾ وَهُوَ فسق الشّرك.