“Eternal Memory” is equivalent to saying “may you ever be in God’s memory.” The Church says this prayer so that the deceased “continue” in God’s memory. Because if God “forgets” us, if He says “I never knew you” (Matt. 7:23), we are led into spiritual extinction. But if He remembers us, then like the thief on the Cross who asked Christ to remember him, we also will live eternally with Him in Paradise.”
(John Sanidopoulos, 21st Century blogger and translator) [Salvation; Prayer for the Departed]