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Post Malone isn't mincing words ... he claims his former tourmate Justin Bieber is wrapped up in the "cult" of Hillsong Church and that the Biebs has donated in excess of $10 million to the megachurch!

In a new Rolling Stone profile, the "Rockstar" rapper's exchange with the RS journalist went like this:

    "When I ask Malone about Bieber, he calls him "a fucking awesome, great dude" and "a genuine friend." He adds, "He's gotten super-religious recently. Real culty." He's referring to Hillsong, a megachurch that Bieber belongs to. "It's not culty!" Ashlen interjects. "It's a total cult," Malone continues. "He's already given them, like, $10 million. Those are the worst kinds of people. I used to be super-religious. I believed in God. But now I see through it. It's nice to support something you believe in, I guess, but people are spending so much money, and God doesn't care that your church has a fucking gold roof." I ask if he's shared these thoughts with Bieber. "We don't talk about this stuff," he says."

A source denied the dollar figure to RS, but still -- makes you think it might be at least close to true, given how tight Bieber and Hillsong founder/pastor Carl Lentz are. When you follow a guru who's paycheck depends on his congregations' tithing, it tends to turn into a Jim and Tammy situation ...