According to the Church, to be a Christian one must believe in Christ and be baptized. See for example CCC 1271, which states that this is the case and 1272 which explains why. In addition, apostasy is the total repudiation of the Christian faith, not just failing to live up to the duties of a Christian, which makes one a sinner (which all Christians are, to one extent or another). Remember too, just being a Christian does not ensure salvation, if we do not repent of our sins. Anyway CCC 2089 defines apostasy.
1271 Baptism constitutes the foundation of communion among all Christians, including those who are not yet in full communion with the Catholic Church: "For men who believe in Christ and have been properly baptized are put in some, though imperfect, communion with the Catholic Church. Justified by faith in Baptism, [they] are incorporated into Christ; they therefore have a right to be called Christians, and with good reason are accepted as brothers by the children of the Catholic Church."81 "Baptism therefore constitutes the sacramental bond of unity existing among all who through it are reborn."82
1272 Incorporated into Christ by Baptism, the person baptized is configured to Christ. Baptism seals the Christian with the indelible spiritual mark (character) of his belonging to Christ. No sin can erase this mark, even if sin prevents Baptism from bearing the fruits of salvation.83 Given once for all, Baptism cannot be repeated.
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."11
