For its Qur’anic evidence, the book cites Qur’an 4:115, which warns about opposing the Messenger and following a path other than that of believers.
«وَيَتَّبِعْ غَيْرَ سَبِيلِ الْمُؤْمِنِينَ»
And follows a path other than the way of the believers.
The usuli inference is that condemning departure from the believers’ path supports the authority of their established common religious path. This is a legal inference from the verse; the full context also concerns opposition to the Messenger ﷺ.