Praise be to Allah, and peace and blessings be upon the Messenger of Allah.
A person is not considered a Muslim merely because of good character or because some of his deeds were righteous. Islam is based on believing in Allah as Lord, Muhammad ﷺ as Prophet and Messenger, and Islam as the true religion, and testifying to that.
As for believing that all religions are correct and that all their followers are upon the truth, this is a false belief, because truth cannot be contradictory and multiple in matters of fundamental creed. Allah تعالى says:
﴿Say: Are those who know equal to those who do not know?﴾ [Az-Zumar: 9]
And He سبحانه says:
﴿Say: Are the blind and the seeing equal? Or are darkness and light equal?﴾ [Ar-Ra'd: 16]
And He عز وجل says:
﴿Say: The impure and the pure are not equal.﴾ [Al-Ma'idah: 100]
However, if a person never received the message of Islam at all, or did not receive it in a manner sufficient to establish the proof against him, then his fate in the Hereafter should not be determined merely by describing him as a disbeliever in this worldly life. Rather, his matter is left to Allah تعالى, who knows his condition best and knows who had the proof established against them and who did not.
Therefore, it is not correct to definitively declare that a particular person who never received the message is among the people of Paradise or Hell. Rather, his matter is entrusted to Allah تعالى, while believing that Allah does not wrong anyone.
And Allah تعالى knows best.