Foster was known to interrupt church services to denounce organized religion's complicity in slavery. In 1841, he was expelled from his Congregationalist church in
Hanover, New Hampshire. In
Portland, Maine in 1842, Foster was wounded in a riot outside a meetinghouse. Pro-slavery supporters wished to prevent Foster and the radical abolitionist
John Murray Spear from speaking. When the mob attacked, Foster took twenty blows to the head and had his coat torn in half. He was pulled from the crowd by women of the Portland Anti-Slavery Society who helped him escape through a back window. Spear was beaten nearly to death on the front steps of the meeting house. Local abolitionists took the men in and tended to their recuperation. In 1843, he wrote the book
The Brotherhood of Thieves; or A True Picture of the American Church and Clergy: A Letter to Nathaniel Barney, of Nantucket and in 1844 Foster's fellow abolitionist
Parker Pillsbury published it through the Boston Anti-Slavery Office. The book went through twenty editions. In 1844, Foster appeared in front of the New England Antislavery Convention holding an iron collar in one hand and iron
manacles in the other. He said "Behold here a specimen of the religion of this land, the handiwork of the American church and clergy." Foster and
Abby Kelley Foster—along with
Sojourner Truth,
Jonathan Walker,
Marius Robinson, and
Sallie Holley—reorganized the
Michigan Anti-Slavery Society in 1853 in
Adrian, Michigan. The state society had been founded in 1836 in
Ann Arbor, Michigan. Foster was called a "
come-outer", a phrase which denoted a person who dissented from religious orthodoxy. More specifically, it meant that Foster would not join a church which held a neutral position on the issue of slavery, and he would not take part in a government that let slavery happen. The phrase was derived from the Bible verse
II Corinthians 6:17, which reads "Wherefore come out from among them, and be ye separate, said the Lord, and touch not the unclean thing; and I will receive you." At the eclectic Free Convention in
Rutland, Vermont, in June 1858, Foster spoke after Pillsbury to say "any law, constitution, court, or government, any church, priesthood, creed, or Bible, any Christ, or any God, that, by silence or otherwise, authorizes man to enslave man, merits the scorn and contempt of mankind." ==Marriage and fatherhood==