it is necessary; one must, ought, has to (an impersonal verb that names a binding necessity laid on a person or event)
This is an impersonal verb: it almost never has a personal subject of its own but reports that something "must" happen, with the person or action that is bound named in what follows. Its core claim is constant everywhere, that a given thing is not optional. What changes from place to place is where the pressure comes from. Sometimes it is the hard logic of a situation, sometimes a duty a person owes, and most weightily in the New Testament it is the settled purpose of God.
The senses below sort the occurrences by that source of the necessity. The line between them is drawn by context, not by the word itself, since the same plain word carries all of them. The reader should not import a heavy sense of destiny where the writer means only that the circumstances left no other choice, nor flatten the loaded uses where a Gospel writer means that God has decreed it.
The heaviest use, and the one that gives the word its theological weight, is the necessity grounded in God's plan: that the Messiah must suffer and rise, that what the prophets wrote must be fulfilled, and that the appointed events of the end must take place. Here "must" is not fate pressing from outside but the unfolding of what God has purposed.