The phrase "separation of church and state" has been attributed to Thomas Jefferson, but did not make it into the Constitution. What the Constitution says, in the First Amendment is:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
It doesn't say separation in there anywhere.
It seems that Rep Adam Kinzinger of Illinois and Rep Lauren Boebert of Colorado have been in a kerfluffle over the issue.
What I know about Kinzinger is that he is a big player in the Jan6 hearings, which tells me that he really does not understand the Constitution. That thing is noting but a show-trial, and ignores little Constitutional concerns like due process and cross-examination.