If an effect follows a cause necessarily and perpetually in the same fashion, explaining why it begins at a distinct time becomes difficult. If a willing agent specifies among possibilities, the temporal occurrence can be understood in relation to volition.
A crucial distinction is between the certainty that an act will occur and its compulsory production without choice. An act's assured occurrence in accordance with will does not itself abolish volition.
Without this distinction, philosophical necessity and theological voluntary agency become confused under the ambiguous word “necessity.”