I disagree. If someone believes that the Jesus Christ of the Gospels is their Lord and Savior, then their prayer can't be useless.
If they believe that the Jesus Christ of the Gospels was just a nice guy, or a manifestation of a shared consciousness or life-force, or a special but non-divine creature then we have an entirely different phenomenon.
This is not a matter of perspective - either you believe Christ is Lord or you don't.
He is the matter of perspective. Some deny one's faith in His finished work on the cross is alone entirely efficacious for salvation. In doing so, they redefine Him. They redefine God.