Snopes debunked this claim.
Carter, briefly, denied visas on the basis of nationality, not religion, strictly during the hostage crisis.
At the same time, he was allowing in many asylum seekers from Iran, including a number of non-Muslim Iranians, but also those who were Muslim Sunnis, seeking refuge from Shia Muslim persecution.
The ban was NEVER religious based, as is the ban proposed by Donald Trump.
As so often happens, from conservatives, who years after the fact try to revise history to put themselves in a better light (civil rights history is just one example), the right is now trying to link themselves to humanitarian Jimmy Carter, winner of the Nobel Peace prize for his work with the leaders of Israel and Egypt, with the Camp David accords. Those accords have held for more than 30 years, and appear likely to be in effect for some time to come.
Clearly Carter held nothing like the anti-Muslim positions and policies advocated by Donald Trump. It is a mark of their innate sense of shame, and fear of criticism and charges of intolerance and bigotry that make conservatives so desperate they are trying to leverage their reputations using Carter.
No one, not liberal or conservative at the time called Carter a racist, because he so clearly was nothing of the kind. The same cannot be said of Trump or far too many of his supporters who are ignorant bigots.