الإرسال واجب، لا بمعنى الوجوب على الله تعالى
Calling the sending of messengers necessary does not mean that God is under an external obligation.
The sending of messengers manifests wisdom, benefit and mercy. 'Necessity' here cannot mean an imposed duty of the sort borne by a creature under a higher authority. God is not subject to another legislator.
The essential distinction is between recognizing the wisdom of an act and claiming that unaided reason can impose that act as a duty upon God. Without it, the possibility, suitability and actuality of revelation become conflated.