Claim. Heaven remains literally empty because the True Parents pattern — the original Adam-Eve royal family of true-love — was never completed in history; no one has entered the actual Kingdom of Heaven.
Elaboration. SMM makes the claim provocatively unequivocal: “The first people who should enter heaven should be the True Parents. If I say that heaven is empty, people will think I am demented. But Jesus could not enter heaven. You cannot go there alone” (2.1. Heaven is the world of the ideal of love, 140-45, 1986.2.1). The doctrinal substance is family-pattern, not membership: “Our original ancestors should have grown through adolescence, established the ideal of love in which God dwelt, become parents and bequeathed the seeds of true love. Then the first place they would have entered would have been heaven. But up until now, there have been no parents embodying God’s love” (ibid). The corollary is that everyone alive today — and historically all the spirits — are in paradise, the middle realm, or hell, never in the actual Kingdom. This sharply distinguishes UC eschatology from any Christian framework that places saints already with Christ in heavenly glory.
See also. csg-jesus-could-not-enter-heaven-and-is-in-paradise