No, I am not! John Tombs and then after him John Bunyan were mixed membership churches and they called it "open communion" in the Associational records. Both were condemned by the Seven churches of London and the associational churches. Tombs complained because the Baptists associations stood opposed to him. So opposed, that any church that practiced mixed membership was not recognzed as a true church and when converted were reconstituted.
John Tombs, although he wrote more on the subject of baptism than any 17th Century Baptist, remained in the Church of England all his life and never founded any churches. [Mike Renihan has written an interesting biography of him]
If the earliest Baptist churches were so opposed to mixed membership, how come Spillsbury remained within a paedobaptist church for some years up until 1633?
I have a suggestion to make as to a possible answer to my last question.
During the 1630s and until the start of the English Civil War in 1642, Archbishop Laud was persecuting all Dissenting churches, and indeed any Anglicans who did not conform to his ultra-Arminian regime. At this time the dissenting churches tended to have good relations with each other as they were all in the same boat. As the Parliamentarians gained the upper hand in the war, the Presbyterians became the dominant party and they became as intolerant and persecuting as the Anglicans. The Congregationalist poet John Milton wrote, "New presbyter is but old priest writ large." It was at this time that the Baptists, not without justice, became very hostile to paedobaptists in general and Presbyterians in particular.
However, after a brief period of religious freedom under Cromwell, the Monarchy was restored in 1660, and shortly after, the persecution began again, of all dissenters, the Presbyterians as well as the others. Once again, in their distress, the dissenters began to grow more together, and when the Baptists came to compile a new confession in 1677, they were not too proud to base it on the Presbyterian WCF. They also dropped their insistence on Closed Communion and left the individual churches to decide for themselves on that matter. Even Kiffin and Knollys, who were such staunch closed-communionists signed up to the new confession when it was finally published in 1689.