John Locke, for "A Letter Concerning Toleration" (1690):
". . .I esteem that Toleration [mutually among Christians] to be the chief Characteristical Mark of the True Church. For whatsoever some People boast of the Antiquity of Places and Names, or of the Pomp of their Outward Worship; Others, of the Reformation of their Discipline; All, of the Orthodoxy of their Faith; (for every one is Orthodox to himself). These things, and all others of this nature, are much rather Marks of Men striving for Power and Empire over one another, than of the Church of Christ. Let any one have never so true a Claim to all these things, yet if he be destitute of Charity, Meekness, and Good-will in general towards all Mankind, even to those that are not Christians, he is certainly yet short of being a true Christian himself."
""It is not the diversity of Opinions, (which cannot be avoided) but the refusal of Toleration to those that are of different Opinions, (which might have been granted) that has produced all the Bustles and Wars, that have been in the Christian World upon account of Religion."
(Hat tip again, to Turner and Quinn)
Part I, here.