That's what they say about everyone else.
My faith tells me I have found the truth and it is in Christ risen.
They say "My faith tells me I have found the truth and it is _________ God."
You have also found your truth in your brand of Christianity.
OTOH, the Orthodox always seem to hedge their bets and say "we really don't know."
Which is true. :)
The fact that others are in error does not necessitate that everyone is in error. There is an optimal reality which we all should be trying to define.
For Christians, that optimal reality is found in Christ through Scripture and is spiritually discerned.
It is real and present and gives evidence of itself by the good fruit it produces.
Paul's confidence was not misplaced nor gnostic. It was tangible because his mind had been quickened by the Holy Spirit to perceive it.
Some get it; some don't.