"Indeed the faithful, the Jews, the Christians and the Sabaeans—those of them who have faith in Allah and the Last Day and act righteously—they shall have their reward from their Lord, and they will have no fear, nor will they grieve." Qur'an 2:62
This verse refers to the followers of previous religions who followed their original religion before distortion and deviation. Every believer must follow the prophet who has been sent after his previous prophet, otherwise he will be disobeying Allah (SWT). The Jewish believer Must believe in Prophet Easa (Jesus) when he was sent, and every Christian believer must believe in Prophet Muhammad (SAWA) when he was sent by Allah (SWT).