Oh jeez, it's hard to pick between these...
"You're not Christians and you never have been Christians, you're unconverted pagans and you need to hear the Gospel!"
"Constantine created the Church and hired some evil men calling themselves 'bishops' to invent the pagan idea of the Trinity and change the meaning of the Bible."
"You guys worship Mary and the Saints!"
"Having icons is idol worship, you know."
"You guys follow the religion created by Paul, not the one founded by Jesus."
"Your religion is only for ethnic tribes like Greeks and Russians."
"You people worship the sun and other pagan gods."
"Your religion is a rip-off of Mithraism/Dionysius' cult/Horus' cult/Gnosticism."
"Jesus never even existed. He was a literary creation made up by some people."
"You're not Christians and you never have been Christians, you're unconverted pagans and you need to hear the Gospel!"
"Constantine created the Church and hired some evil men calling themselves 'bishops' to invent the pagan idea of the Trinity and change the meaning of the Bible."
"You guys worship Mary and the Saints!"
"Having icons is idol worship, you know."
"You guys follow the religion created by Paul, not the one founded by Jesus."
"Your religion is only for ethnic tribes like Greeks and Russians."
"You people worship the sun and other pagan gods."
"Your religion is a rip-off of Mithraism/Dionysius' cult/Horus' cult/Gnosticism."
"Jesus never even existed. He was a literary creation made up by some people."