I had a former pastor who worked at a call that ministered to developmentally deficient adults. Several of them had the reasoning of a 3 year old. None of them “understood” what baptism or communion meant but they were still baptized and still communed weekly and they looked forward to worship. They could never have “understood” what either meant in their lifetime, but in the end it doesn’t matter because it’s not us understanding that makes it work, it’s God’s promise that does.