It is evident reading the Quran that the Prophet Muhammadﷺ does not only speak in the name of God but that he is telling the Word of God as he has heard and understood it.
Those who do not believe him will not accept this, saying it only testifies itself.
But also the Torah testifies itself because it is the only ancient source stating that what is contained in it is the message God gave to Moses (or to Abraham and his successors before him).
If they only accept the Bible: Of course it can not testify that God spoke to Muhammadﷺ because all the Bible has been written before.
And how are they behaving now towards a Book which has come to them
from Allah? Inspite of the fact that it confirms the Scriptures which
they already possessed. and, inspite of the fact that, before it came,
they used to pray for a signal victory over the disbelievers, they
rejected it when it came, although they recognized it.
But if they do not accept the Quran, they can accept this ;-)
So let the people of the Gospel rule according to what God revealed in
it. Those who do not rule according to what God revealed are the