«قُلْ إِنْ كَانَ لِلرَّحْمَٰنِ وَلَدٌ فَأَنَا أَوَّلُ الْعَابِدِينَ»
Say: If the Most Merciful had a son, I would be the first of the worshippers.
The use of in does not suggest real uncertainty about a divine son. The hypothetical may be explained as a way of confronting an opponent’s claim and exposing its weakness.
The complete interpretation depends on the reading of al-ʿabidin and context; it should not be reduced to a mechanically assumed translation. The rhetorical point is that a rejected proposition can be temporarily adopted as an argumentative premise.