Can such a person still relate to God on a deep level in the
spiritual/soul plane though facing limitations in the capacity of
their earthly mind?
Blesses are the pure in heart: for they will see God. (Matt 5:8)
God does not judge us by what we know. He judges us by what we love, and how much we love.
We are saved or damned according to what we love. If we love God, we
shall ultimately get God: we shall be saved. If we love self in
preference to God then we shall get self apart from God: we shall be
damned. (Frank Sheed, Theology and Sanity)
Scripture isn't clear as to how exactly how a mentally handicapped person is saved (using "handicapped" in the modern sense of the word). From the Apostolic period to relatively recent times mental disabilities and vegetative states of existence were addressed more as some sort of retribution from God. Today the science of psychology and medicine have revealed much more depth about what causes these afflictions.
The closest Sacred Scripture comes to expliclty addressing the handicap of ingnorance is when Jesus speaks of children.
Luke 18:17 - Verily I say unto you, Whosoever shall not receive the
kingdom of God as a little child shall in no wise enter therein.
Matthew 18:3 - And said, Verily I say unto you, Except ye be
converted, and become as little children, ye shall not enter into the
kingdom of heaven.
Mark 10:13-16 - And they brought young children to him, that he should
touch them: and [his] disciples rebuked those that brought [them].
Matthew 21:16 - And said unto him, Hearest thou what these say? And
Jesus saith unto them, Yea; have ye never read, Out of the mouth of
babes and sucklings thou hast perfected praise?
The Code of Canon Law of the Roman Catholic Church has no official policy about the sacraments and the developmentally disabled as such. The nearest parallel can be found in the principles relating to infant communion, specifically, the age of reason. According to the 1913 Catholic Encyclopedia article on Communion of Children:
In the best-supported view of theologians this phrase means, not the
attainment of a definite number of years, but rather the arrival at a
certain stage in mental development, when children become able to
discern the Eucharistic from ordinary bread, to realize in some
measure the dignity and excellence of the Sacrament of the Altar, to
believe in the Real Presence, and adore Christ under the sacramental
Canon 913 states:
§1. The administration of the Most Holy Eucharist to children requires
that they have sufficient knowledge and careful preparation so that
they understand the mystery of Christ according to their capacity and
are able to receive the body of Christ with faith and devotion.
§2. The Most Holy Eucharist, however, can be administered to children
in danger of death if they can distinguish the body of Christ from
ordinary food and receive communion reverently.
Outside of Canon Law, the United States Conference of Catholic Bishops' Guidelines for the Celebration of the Sacraments with Persons with Disabilities states that:
The criterion for reception of holy communion is the same for persons
with developmental and mental disabilities as for all persons, namely,
that the person be able to distinguish the Body of Christ from
ordinary food, even if this recognition is evidenced through manner,
gesture, or reverential silence rather than verbally. Pastors are
encouraged to consult with parents, those who take the place of
parents, diocesan personnel involved with disability issues,
psychologists, religious educators, and other experts in making their
judgment. If it is determined that a parishioner who is disabled is
not ready to receive the sacrament, great care is to be taken in
explaining the reasons for this decision. Cases of doubt should be
resolved in favor of the right of the baptized person to receive the
sacrament. The existence of a disability is not considered in and of
itself as disqualifying a person from receiving the eucharist.
It is essential that all forms of the liturgy be completely accessible
to persons with disabilities, since these forms are the essence of the
spiritual tie that binds the Christian community together. To exclude
members of the parish from these celebrations of the life of the
Church, even by passive omission, is to deny the reality of that
community. Accessibility involves far more than physical alterations
to parish buildings. Realistic provision must be made for persons with
disabilities to participate fully in the eucharist and other
liturgical celebrations such as the sacraments of reconciliation,
confirmation, and anointing of the sick (Pastoral Statement of U.S.
Catholic Bishops on Persons with Disabilities, November 1978; revised
Speaking from a parent's perspective with a child with autism, I have learned more from my son about living the Gospel by loving unconditionally than I ever could from any of the best theologians. I've observed from working with disabled children in the field of Psychology that the more severely handicapped a child is, the more beatific their disposition - at least in most cases.
For more detailed resources about the implementation of programs that draw people with disabilities visit the National Catholic Partnership on Disabilities.