It should be - it's a matter of record that President Obama is a Christian, and it's a pre-requisite under the law that any Presidential candidate must be born in the country.
Yet another example of Trump shooting himself in the foot. Until quite recently, Trump was of the opinion that Obama wasn't an American, although he eventually had to admit defeat on that issue.
But perhaps it explains why he didn't immediately put the hecker right. Its a case of "let the idiot heckler do my job for me "
If this man ends up in the White House, God help us all !