With God all things are possible. Why do these people doubt?
It is not a matter of a lack of faith, it is a matter of a lack of Scriptural support for a doctrine. If there is no Scriptural support for a particular doctrine then there is no reason to believe that doctrine. The Bible is silent regarding the exact nature in which the communion is the body and blood of Christ. Transubstantiation (and consubstatiation) is, at best, human speculation. Yet people have been burned at the stake over this issue - an issue of human speculation!
Incidentally, according to the Apostles themselves, there are certain things which are, in fact, impossible for God.
"... in which it was impossible for God to lie,..." - Heb. 6
"... He cannot deny Himself...." - 2 Tim. 2
"... for God cannot be tempted with evil..." - James 1
"... which God, that cannot lie, promised before the world began;" - Titus 1
I find it somewhat curious that some Protestants, who take the scripture sooooo literally, seem to fall down when Jesus says "this IS my body"