Speaking on a CNN show, Jerushah Armfield said the president's actions actually represent the opposite of Jesus' message.
She said: "My president doesn't have to be a Christ follower. My president doesn't have to be a Christian.
"However, I think my rub is when the evangelicals are coming up and posing [Trump] as the godly candidate."
Her uncle Franklin Graham is one of many popular evangelists that are supporters of President Trump.
Armfield said the US president's actions had been the opposite of Jesus'.
When asked by anchor Pamela Brown if she believes the president is a man of strong faith, she replied, "absolutely not".
She went on to explain: "I think we can look at his life, look at the intolerance that he's spoken, I think that my Jesus that I follow was really somebody who fought for the outliers, and I think that Trump has actually done the opposite - kind of ostracizing them."
Armfield recommended her Trump-supporting uncle "stick to" ministry instead of politics.
She said: "I think my uncle is an incredible - he has an incredible humanitarian ministry that's been on the front lines often before a lot of ministries have been there.
"I think he probably needs to stick to doing that. I think that he believes that he's speaking to a larger audience than he is.
"I think the audience he was once speaking to is starting to migrate to a little more progressive open mindedness."
She also slammed supporters of former Alabama Supreme Court judge Roy Moore.
Armfield said she would support the term evangelical being changed as it had "started to really represent... a branch of Christians that seemed to be a little more conservative and a little bit more hypocritical, a little bit more willing to compromise on the personal morals of a candidate in lieu of what politically they could gain for their party".
