The futility of belief after witnessing punishment

أَثُمَّ إِذَا مَا وَقَعَ ءَامَنتُم بِهِۦٓ ۚ ءَآلْـَٔـٰنَ وَقَدْ كُنتُم بِهِۦ تَسْتَعْجِلُونَ

Then is it that when it has [actually] occurred you will believe in it? Now? And you were [once] for it impatient.