If God made us to know him, love him and serve him, why did he make some of us who will never be able to do this because they do not have all their faculties?
Your question seems to define "knowing" in merely intellectual terms. Yet knowing, in terms of faith, is something richer than a mere intellectual grasping of God.
Further, we cannot know fully the inner life of the mentally disabled. The same can be said for infants and very young children. I have a small memory of my very early childhood when I was perhaps five years old and that memory is of great intimacy with God, who spoke to me simply and with love, and I to Him. As I grew older, and my brain grew "bigger" my heart also seemed to diminish and I lost that experience of intimacy with God. I have spent these, my later years, trying to recover that early intimacy.
I do not offer this memory as proof that little children, or, by analogy, the mentally disabled, all have this intimacy, but only to indicate that there are mysteries in how God relates to us that cannot be simply reduced to high intellectual knowing.
It would seem rather that God relates to us in ways appropriate to our state. It would also seem we should at least be open to the possibility that the mentally disabled may have an even greater intimacy with God than we of "able mind" can only admire as we seek to become more "like little children, so as to enter the Kingdom of God" (cf Mark 10:15).
"Building our Catholic faith one question at a time."