If the reference to the barbarity of theistic belief, as noted in the question, is the only requirement for condemning such a belief then there might be cause to condemn those political aspirations that have caused far greater barbarity. Communism/Socialism/ Marxism has been the cause of the greatest number of human deaths and, by far the greatest cause of human suffering. http://reason.com/blog/2013/03/13/communism-killed...
The Jefferson quote is not quite complete and taken out of context. Below is the full quote, which I wholeheartedly agree with, but even that Jefferson promotes a rigid separation he recognizes the right of the people to believe in a god, and does not condemn it as a matter of opinion. It is the action of theists that concerns Jefferson, not the belief.
"Believing with you that religion is a matter which lies solely between Man & his God, that he owes account to none other for his faith or his worship, that the legitimate powers of government reach actions only, & not opinions, I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should "make no law respecting an establishment of religion, or prohibiting the free exercise thereof," thus building a wall of separation between Church & State. Adhering to this expression of the supreme will of the nation in behalf of the rights of conscience, I shall see with sincere satisfaction the progress of those sentiments which tend to restore to man all his natural rights, convinced he has no natural right in opposition to his social duties." https://www.loc.gov/loc/lcib/9806/danpre.html
The claim by the author of the question that, " Christian fanatics lobbying government to massacure gays,": precisely where can evidence of this be found?