In the 1990s, at the end of the second millennium, it was widely feared that the "Y2K" software glitch would prove literally apocalyptic. Levin had set Rosemary's Baby in the span of 1965-1966; that meant titular 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 – 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.