Campbell allowed for a person who had mistaken the outward baptism to posesses the inward, so that, he would much rather fellowship a non-sectarian paido-baptist than a sectarian immersionist any day of the week. He stated that he did indeed, find true Christians scattered amongst all the sects, yet called for them to come out of them and be "Christians only." Campbell had been convinced that the unity of the Church would speed the evangelization of the world, which itself would hasten the advent of the millennium (Campbell was postmillennial while Stone, Scott, Lipscomb, Harding, etc. were premil.). Of course Campbell was disappointed that it didn't work out that way. He deeply lamented the sectarian spirit of many of his own followers, lamenting in the 1841 Millennial Harbinger that his brethren had some two dozen newspapers and from many of them, it was difficult to ascertain what the aims and goals of the Movement were.
The Lunenburg Letter was occasioned, possibly by the controversy started by Dr. John Thomas, over whether Baptist baptisms or other immersions done without knowledge of or reference to, remission of sins, wwere valid. Thomas said No, and urged rebaptism, while Campbell said Yes, citing his own baptism as an example. He and several members of his family had been immersed by Baptist Elder Matthias Luce in 1812 without any reference to remission of sins. So if such baptisms were invalid, so was Campbell's. Yet Campbell never felt it necessary to be rebaptized.
But both Thomas and Alexander Campbell deeply regretted their estrangement with their Baptist brethren (their words). As late as 1866, when Campbell was on his deathbed, he was praying for an official union with the Christians/Disciples and the Baptists of that part of Virginia. The Campbells originally intended to remain afiliated with the Baptists so long as they were free to study and preach their beliefs. They eventually felt compelled to separate from the Baptists when they were pressed to adopt the Philadelphia Confession of faith.