Giving a statement at Wellspring Church in Ferguson, Maria Chappelle-Nadal said she had made "a mistake" and was "owning up to it".
The post, which was later deleted, was written in response to another comment which called for the president to be impeached. It said: "No. I will. I hope Trump is assassinated!"
There have been numerous calls for her to resign, while the US secret service also investigated the comments.
Ms Chappelle-Nadal has also received death threats following the post.
Addressing reporters at the church on Sunday, she said: "I made a mistake, and I'm owning up to it. And I'm not ever going to make a mistake like that again.
"I have learned my lesson. My judge and my jury is my Lord, Jesus Christ.
"President Trump, I apologise to you and your family."