قُلْ أَرَءَيْتُمْ إِن كَانَ مِنْ عِندِ ٱللَّهِ ثُمَّ كَفَرْتُم بِهِۦ مَنْ أَضَلُّ مِمَّنْ هُوَ فِى شِقَاقٍۭ بَعِيدٍ
"Say: And what if it is from God, and you have denied it? Who is further astray than one who is in a far-off breach?"
The verse does not set up a claim — it sets up a possibility. In kāna min ʿindi'llāh — "and what if it is?"
I record this as an observation about method: the addressee is not asked to accept the other's position; he is asked to take his own possibility into account. In the fifth verse of the sūra the other side had closed the argument (fa'ʿmal innanā ʿāmilūn — you work, we are working). This verse reopens the closed argument with a question.
شِقَاق — root ش-ق-ق: to split, to cleave in two. The root was treated at al-Qamar 54/1 (wa'nshaqqa'l-qamar) and in al-Inshiqāq. Shiqāq is the falling of two sides into separate halves.
And the adjective baʿīd (far): the breach not only exists, its distance too is great. The same adjective was treated at Qāf 50/3 (rajʿun baʿīd) — there those who denied counted the resurrection "far off". Here what is far off is their own condition.