Though Oncken lived in
Scotland and was converted in London, there is no existing evidence of any contact with the English Baptists. The evangelical Christianity of Scotland was influenced by the
Haldane brothers, and it is known that at least later Oncken made contact with them. Oncken apparently arrived at a Baptist position on
believer's baptism (and
baptism by immersion) by degrees through personal Bible study. By 1829, though still a member of a church that taught infant baptism, he appears to have decided against it, for in that year he refused to present his child for the ceremony. He discussed the baptism issue with T. W. Matthews (his pastor), C. F. Lange (one of his first converts) and others. Matthews became convinced that he should be baptized by immersion, resigned from his church and traveled to England to be baptized. In 1829, Oncken corresponded with
James Haldane of Scotland. Haldane recommended that Oncken baptize himself, as
John Smyth had done. Oncken thought this was without Biblical authority, and said of Haldane's advice, "Even great men are able to err." Oncken also corresponded with
Joseph Ivimey. Ivimey invited him to come to London and receive believer's baptism. Oncken had also told his story to Calvin Tubbs, a sea captain. Tubbs told Oncken's story to the (American Baptist)
Triennial Convention. In 1833,
Barnas Sears, a professor at Hamilton College, visited Germany for studies. Having heard the story, he made it a point to find and speak to Oncken. By 1834, Oncken had made a final decision. Sears traveled from Halle, where he was studying, to Hamburg, and baptized Oncken, his wife and five others in the Elbe on 22 April. The baptism was performed at night. The next day, Sears founded the first
Baptist church in
Hamburg. The church quickly grew to 68 members by 1836, but after that year persecution temporarily halted its growth. Though the Baptists initially engaged in performing baptisms at night, in 1837 Oncken began to baptize openly. That year, he baptized fellow German Baptist pioneer
Gottfried Wilhelm Lehmann on
Pentecost Sunday. German authorities shared the traditional Christian opposition to
rebaptism; they also felt that people being dipped in the river was an offence to public morals. The civil authorities gave them peace for a while after the Hamburg fire of 1842, due to the help the Baptists gave to the people of the city. In 1848, Oncken was involved in founding , the first Baptist paper published regularly in Europe. In 1849, he began to hold organized classes with his students, and in 1880 a four-year seminary was established in Hamburg. He was also influential in the German Baptist confession of faith issued in 1847, and the organization of the
Union of Baptist Congregations in 1849. Oncken also played a role in the conversion of
Anders Wiberg, a key figure in the early Swedish Baptist movement, who came to agree with Baptist teachings regarding baptism after a visit with him. ==Relations with Mennonites==