The first and oldest educational institution of the Southern Baptist Convention disclosed in a report Wednesday that its four founders together owned more than 50 slaves, part of a reckoning over racism in the nation’s largest Protestant denomination. The 71-page report released by the Southern Baptist Theological Seminary is a recitation of decades of bigotry, directed first at African slaves and later at African-Americans. Beginning with the founding of the seminary in Greenville, S.C., in 1859, the report found that the school, with few exceptions, backed a white supremacist ideology.