With Mitt Romney campaigning as one of the GOP front-runners for president, his Mormon religion is coming under increasing scrutiny.

The Mormon question is especially critical to a key part of the GOP base -- conservative evangelical Christians. Texas Pastor Robert Jeffress made national headlines recently when he labeled Mormonism a cult after introducing Texas Gov. and presidential candidate Rick Perry.

Now Mark Driscoll, pastor of the high-profile Seattle-based Mars Hill Church, has added his voice to the fray -- agreeing that Christians should consider Mormons as members of a "cult" and not "brothers and sisters in a common faith."

Writing on his website Tuesday, Driscoll goes on at length about the various definitions of a cult. He is careful to say that Mormonism has "outgrown" the popular culture and sociological definitions of a cult because of its enormous growth and general acceptance in the nation.

But, Driscoll writes, Mormonism "is most certainly a cult theologically speaking because it deviates substantially from historic Orthodox Christian belief about essential issues related to God, humanity, and salvation." And it's not just that it differs, he says, but also that "it claims Christianity while subtly subverting it in both practice and theology."

"Because it claims to be Christian, uses Christian language, but is antithetical Christianity, it must be labeled a cult theologically," Driscoll writes.

He concludes:

"So what do we make of all this? Simply said, by the theological definition, Mormonism is a cult."

"As the presidential race heats up and the prospect of a practicing Mormon as a viable Republican candidate becomes more a reality, there will be continued effort to bring Mormonism into the center of Christian orthodoxy. Thus, it's important to understand what the cult of Mormonism teaches, to understand that it's antithetical to Christianity, and that, while it's certain there are some Christians in the Mormon church who love the Jesus of the Bible and don't understand or agree with what their church teaches, the Mormon church could never be considered orthodox unless it made some serious and massive changes to it's theology..."

"Many Mormons are good neighbors, friends, and fellow citizens. But, we cannot go so far as to call them brothers and sisters in a common faith. To do so is to not only confuse real Christians, but to also diminish the importance of lovingly speaking with Mormons about the errors of their belief in hopes of seeing them come to know the real God of the Bible and avoiding eternal damnation for worshiping a false god."