The word must is important. The concern of this book is not with things which simply will happen, as though the future were of absorbing interest in itself.  It is rather taken up with things which must happen, the outworking of the divine will.  God is in supreme control.  John is not writing about matters of chance, but about events which will certainly occur, for they are part of the divine plan.

– Leon Morris on Revelation 4:1 in The Revelation of St. John (TNTC)