Ever since the Last Supper, Catholics have pondered what happens during the Mass when they believe the bread and wine become the Body and Blood of Jesus.

"Because Christ our Redeemer said that it was truly his body that he was offering … it has always been the conviction of the Church … that by the consecration of the bread and wine there takes place a change of the whole substance of the bread into the substance of the body of Christ our Lord and of the whole substance of the wine into the substance of his blood," proclaimed the Council of Trent, after the Protestant Reformation.

"This change the holy Catholic Church has fittingly and properly called transubstantiation. The Eucharistic presence of Christ begins at the moment of the consecration and endures as long as the Eucharistic species subsist."

Believers approach this mystery with the greatest care and respect. This may be hard for children to grasp as they prepare for First Communion.

Now imagine trying to teach this core Catholic doctrine to persons -- young and old -- who have mental and physical disabilities that make it hard, or impossible, for them to acknowledge what is happening in the Mass.

"Because we believe Holy Communion is the Body and Blood or our Lord, we want to be very careful about this," said Father Matthew Schneider, who is known to his Twitter followers as @AutisticPriest.

"This isn't a theology test. No one needs a theology degree to take Holy Communion. We simply need to make sure that they know this is an act in a church rite -- that they are not eating ordinary food like at home. We're trying to find out if they have a basic understanding of what's happening."

Under Catholic canon law, children can be given Holy Communion "if they can distinguish the body of Christ from ordinary food and receive communion reverently."