In the larger sense, all of those persons who are baptized are members of the Church founded by Christ. Its more reserved interpretation includes all those who are baptized and united with the Roman Catholic Church.
In his encyclical, On the Mystical Body of Christ (1943), Pope Pius XII declared: “Actually, only those are to be included as members of the Church who have been baptized, and who profess true faith, and who have not been so unfortunate as to separate themselves from the unity of the Body or have been excluded by legitimate authority for grave faults committed.”
It is apparent from this statement that the requisite conditions are the ritual and sacramental (baptism), and the declaration or profession of faith in Christ and unity with the Church.
Sin does not exclude one from membership, except sin against faith or an action that would cause the Church to declare the membership ended, such as in excommunication. Sins against faith and Church may be done away with through repentance and the reception of the Sacrament of Reconciliation. (7:19)