Private jets, giant mansions and extravagant lifestyles. No, we're not talking about Donald Trump. Plenty of televangelists have created vast empires just by preaching and working up a religious fervor on their weekly programs. On Sunday's episode of HBO's "Last Week Tonight with John Oliver," the host took a closer look at some of these high-flying preachers and examined how tax laws have helped them keep large sums of money.

Oliver started the segment with a look at Creflo Dollar, founder and pastor of the nondenominational World Changers Church International, based outside Atlanta. The pastor made news in May when he urged followers to help him raise $65 million so he could purchase a Gulfstream jet. Dollar said he needed a new jet because his current private plane was getting too old and unsafe. The plane was in addition to Dollar's multiple residences -- including a $2.1 million home in New York.

Dollar isn't the only televangelist living in luxury, Oliver said, adding that TV preachers have made it a habit to flaunt their wealth.

But the pastors argue that private jets, while lavish, are often necessary to help them spread their ministries -- they tend to have many speaking engagements and appearances around the world. However, some -- like Kenneth Copeland, leader of the Kenneth Copeland Ministries in Texas -- have used them for private trips as well.

Oliver also explored how these pastors can earn so much money by teaching the "prosperity gospel," meaning the more money followers give to the church, the more money they'll probably make in the future -- at least that's the pastors' theory. But for some individuals, too much giving can lead to personal debt or an inability to pay for potentially life-saving medical treatment, Oliver noted.

Much of the money generated by these churches -- through donations and other means -- is tax-exempt, Oliver observed. That's because the IRS has very loose standards regarding religious-oriented nonprofit organizations or churches. As the IRS sees it, practically any opinion or belief -- as long as it's not illegal -- can be a religion, Oliver argued. The agency audited just one church in 2014, and just two in 2013, he noted.

Perhaps the best part of the segment was Oliver's correspondence with Robert Tilton, creator of the religious-themed TV program "Success-N-Life," an infomercial-style religious program that was temporarily taken off the air in the 1990s after ABC News questioned Tilton's fundraising practices.

Oliver's segment began hilariously, but ultimately the host grew quite angry at the televangelists and announced he had created his own church to expose their predatory ways. You can watch the "Last Week Tonight with John Oliver" segment below.