There is a difference between man's heart (biblical mind) and intellect (brain.) Although one's intellect may be weak, impaired and wasting away, (or even immature), based on the biblical distinction of what the heart/mind is, that person can still glorify God and reflect the light of Christ.
Therefore we do not lose heart. Though outwardly we are wasting away, yet inwardly we are being renewed day by day. (2 Corinthians 4:16)The heart can be taught (2 Timothy 3:16) and it must be made new by redemption grace (Ps 51;10) through faith, provided by the Holy Spirit - this is the power behind true knowledge and understanding. In that faith there is hope!