If your parents truely believe in a certain fath then they believe that is the fullest and most complete truth available. If they believe that, why would they ever feel the need to say "But that religion (which they think is maybe only partially true/complete) is just as good as ours" ?
Granted this scenario is based on said parents truely believing in thier faith.
So really taking in what "Faith" means it is completely logical that the only thing they would teach as truth is thier own beliefes.
"Anybody who goes to bed the same day they got up is a quitter."
Achewon Nutiket*
*Spiritual Guard
† RIP Kory †