﴿ وَلَوْ جَآءَتْهُمْ كُلُّ ءَايَةٍ حَتَّىٰ يَرَوُا۟ ٱلْعَذَابَ ٱلْأَلِيمَ ﴾

“even though every sign [of the truth] should come within their ken - until they behold the grievous suffering [that awaits them in the life to come].”

though every sign come to them until they see the painful chastisement at which time it their belief will be of no use to them.