إِنَّ ٱلَّذِينَ قَالُوا۟ رَبُّنَا ٱللَّهُ ثُمَّ ٱسْتَقَٰمُوا۟ فَلَا خَوْفٌ عَلَيْهِمْ وَلَا هُمْ يَحْزَنُونَ
"Surely for those who say 'our Lord is God' and then stand straight there is no fear; nor shall they grieve."
In Arabic thumma, unlike fāʾ, conveys a distance between the two. That is, there is a stretch of time between the saying and the standing straight: saying is done in a moment, standing straight lasts.
ٱسْتَقَٰمُوا۟ — Form X, root ق-و-م: to stand straight, to continue without bending. The root's connection with qayyūm was treated in al-Baqara at 2/255: to stand, and to hold standing.
I record this as my own reading: the verse sets two conditions, and the second is spread over time. This is also what will be related through the rest of the sūra: when ʿĀd saw a cloud (24), when a man reaches the age of forty (15), when the jinn heard a text (29) — all of them are about what is done beyond the first moment.
فَلَا خَوْفٌ عَلَيْهِمْ وَلَا هُمْ يَحْزَنُونَ — this construction occurs in many places in the Qurʾān, and it closes off both directions of time: khawf belongs to the future, ḥuzn to the past. That is, neither what waits ahead nor what is left behind.