"I saw Father Adam and Abraham; and my father and my mother; my brother Alvin, that has long since slept; And marveled how it was that he had obtained an inheritance in that kingdom, seeing that he had departed this life before the Lord had set his hand to gather Israel the second time, and had not been baptized for the remission of sins." — Joseph Smith1
During a vision, Joseph Smith saw the celestial kingdom, which he described vividly with gates like circling flames of fire and streets that appeared paved with gold.2,3 He beheld God the Father and Jesus Christ seated on blazing thrones, surrounded by Old Testament prophets like Adam and Abraham. He also saw his family members, including his brother Alvin, who had died before the Church had been established.2
Joseph was initially perplexed about how Alvin, who died unbaptized, could inherit celestial glory. During the vision, the Lord's voice explained that all who die without a knowledge of the gospel, but who would have embraced it if given the opportunity, are heirs to the celestial kingdom. This doctrine was particularly comforting concerning the fate of small children and those like Alvin who passed away before having the chance to accept the gospel through earthly ordinances.2
This vision is now found in Doctrine and Covenants section 137.3