In the 1990s – the tail end of the second millennium – it was widely feared that the Y2K software glitch would prove literally apocalyptic (much as it's now feared AI could). Levin had set Rosemary's Baby during the span of 1965-1966; that meant Andrew Woodhouse would turn 33 in 1999 — the same age at which Christ is believed to have died, and been resurrected.
This coinciding of the book's timeline with that of society's – as it hurtled towards a new millennium, and its possible demise – gave Levin a special impetus to write Son of Rosemary, and set it in 1999 – on the teetering cusp of humanity's collapse, or its emergence into a new era.
Son of Rosemary picks up with Andy having established a Christ-like ministry of his own to the world – as Rosemary questions if her efforts at nurturing him have served to overcome his dark origins – or if he's truly his father's son.