What does the Bible tell us about those with dementia or Alzheimer's? Will we see them in heaven? How do we know if they still believe in God's saving grace?
The Bible says nothing directly or explicitly about dementia and Alzheimer’s. Ironically, the strong majority of references to the aged, a frequent target of these diseases, view old age as a blessed time of life. The subject of the loss of mental capabilities through diseases like dementia and Alzheimer’s does not surface in the Scriptures. I have heard the words of Jesus to Peter in John 21:18 applied to people suffering from this kind of malady, but in context the words refer to physical martyrdom, not disease or a loss of physical or mental capabilities.
Your primary concern is the really important one: do these diseases rob a believer of saving faith in Jesus Christ or give us cause for concern about their faith-life until death? Here we happily report that saving faith is not at all the same as mental knowledge, or the ability to memorize and articulate information about the Savior, or even the power to memorize and repeat truths about Jesus to other people. Faith is ultimately the product of the Holy Spirit who creates and maintains reliance on Jesus Christ in a person’s heart.
Usually this work of the God involves the accompanying use of a person’s mental and emotional abilities, as in the study and learning of the Word of God. The Holy Spirit uses the written and spoken gospel to give and sustain saving faith. But he is also fully able to work saving faith in infants and small children through the instrument of Baptism; and this divine, gracious working is beyond our ability to comprehend. It is also beyond our ability to discern or recognize with our senses. But because God promises that infants and little children can indeed be brought to saving faith (see Matthew 18:6,10 and Acts 2:38-39) we take him as his word and entrust the little ones to his care. Similarly, we do the same with those who have been rendered unable to express themselves or articulate their Christian faith as they once could.
Our convictions and comfort rest on divine promises rather than empirical evidences that this is so. God is faithful and will not abandon his people despite the ravages of disease.