John Wesley published his sermon ‘Catholic Spirit’ in 1750, after he and his preachers had experienced persecution by Church leaders. Wesley stressed that persecution stemmed from lack of tolerance, and one of the reasons for this was the absence of liberty of thinking in the Church. In order for liberty of thinking to be practised, one had to be able to doubt one's own opinions, thereby accepting the limitations of one's knowledge. Most of this sermon, now lauded for its ecumenical brilliance, asserts that such acceptance provides space for tolerance. This tolerance leads to Christian unity. In addition to exploring the sermon, this essay addresses An Answer to the Rev. Mr. Church's Remarks on the Rev. Mr. John Wesley's Last Journal (1745), Letter to a Roman Catholic (1749) and Wesley's correspondence with Gilbert Boyce (1750). The argument thus provides an example of how doubt contributed to the Methodist emphasis on tolerance.