Reason and free inquiry are the only effectual agents against error. Give a loose to them, they will support the true religion, by bringing every false one to their tribunal, to the test of their investigation. They are the natural enemies of error, and of error only.
 Thomas Jefferson, Notes on the State of Virginia (1785). copy citation

Context

“If it be said, his testimony in a court of justice cannot be relied on, reject it then, and be the stigma on him. Constraint may make him worse by making him a hypocrite, but it will never make him a truer man. It may fix him obstinately in his errors, but will not cure them. Reason and free inquiry are the only effectual agents against error. Give a loose to them, they will support the true religion, by bringing every false one to their tribunal, to the test of their investigation. They are the natural enemies of error, and of error only. Had not the Roman government permitted free inquiry, Christianity could never have been introduced. Had not free inquiry been indulged, at the æra of the reformation, the corruptions of Christianity could not have been purged away.” source