As the debate about religion and the 2016 US presidential election heats up, a megachurch pastor from Texas is calling on Christians to consider the candidates' faith in making their choice for the US presidency.

Pastor Robert Jeffress, who leads the 11,000-member First Baptist Church in Dallas, encouraged Christians to use a "religious litmus" test when picking which presidential candidate to support for the upcoming election.

"To say that a candidate's faith doesn't make any difference is absolutely ridiculous," Jeffress told OneNewsNow in an article posted on Tuesday.

"I mean, our faith gives us our worldview. It's the essence of who we are," the pastor added.

Jeffress also defended Republican presidential candidate Dr. Ben Carson, who drew criticism after suggesting that a Muslim should not be elected as US president.

The pastor said Carson was only standing by his Christian faith. Jeffress also said if a Muslim eventually becomes a US president, he or she should publicly denounce "radical Islam" and the Quran, the Islamic Holy Book.

"Frankly, any so-called Muslim who renounces the Quran and Muhammed is probably no more a Muslim than a Christian who renounces the Bible and Jesus Christ is a Christian," Jeffress said.

Carson's recent remarks against Muslims, however, did not sit well with his fellow Republican presidential candidates, such as South Carolina Senator Lindsey Graham.

Graham said Carson's controversial statement highlighted the fact that the retired neurosurgeon is not yet ready to be US President.

"This shows that Dr. Carson is not ready to be commander-in-chief. Mr. Carson may be a good doctor, but he is not ready to lead a great nation," the senator said.

Texas Senator Ted Cruz, meanwhile, commented that religion should not be a consideration in choosing the next American leader.

"You know, the Constitution specifies there shall be no religious test for public office and I am a constitutionalist," Cruz said.