Does Qur'an 2:62 confirm the validity of other religious doctrines?
"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
Another thing to consider is that the verse is about people, not doctrines. That is, the verse is not offering a view on Judaism, Christianity, or Sabaeanism.
Rather, the point of the verse is that, in the afterlife, individuals will be judged according to their inner and outer realities and whether they have purified or destroyed their souls, not their worldly identities. It is not enough simply to say "I'm a Christian!" or "I'm a Muslim!" and expect salvation.
Also, the verse is advising us not to be arrogant about our religious identity.
