The Catechism on apostasy:
2089 Incredulity is the neglect of revealed truth or the willful refusal to assent to it. “Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Roman Pontiff or of communion with the members of the Church subject to him.”
“We are saved by grace through faith.”
Faith is necessary for salvation, so without faith we are damned. Apostasy is total rejection of the faith, therefore apostates are damned (without repentance).
The damned are not in the Communion of Saints, therefore apostates are not in the Communion of saints.
Also, in order to remain Catholic in salvation with the Church one must reject falsehood in order to avoid apostasy.