Are Roman Catholics Christians? They are if they have trusted in Jesus alone for the forgiveness of their sins. However, if they believe they are saved by God's grace and their works, then they are not saved--even if they believe their works are done by God's grace--since they then deny the sufficiency of Christ's sacrifice.
Being a Christian does not mean being a member of the Roman Catholic Church. It means being a member of the body of Christ, which is accomplished by faith and trust in Jesus alone for the forgiveness of your sins. It means you do not add your works to His work. Sincerity doesn't forgive sins. Membership in a church doesn't forgive sins. Doing works of penance doesn't forgive sins. Praying to Mary doesn't forgive sins. Forgiveness is received in the faithful trust and acceptance of Jesus' sacrifice on the cross. You must trust Jesus, God in flesh, for the forgiveness of sins--not a man-made ritual and certainly not the catholic saints. Even though the Roman Catholic Church affirms the Trinity, the deity of Christ, and His physical resurrection, it greatly errors in its doctrine of salvation by adding works to salvation.