There was a time during the colonization of America, and earlier in England, that Christians considered it their right to enslave non-Christians of any race. The slaves could gain their freedom by converting to Christianity. Indentured servitude was also common. A wealthy person would give money or help in exchange for a specific amount of the debtor's labor, commonly seven years. A Christian could not be held as a servant for life except as punishment for criminal acts.
Antonio Johnson (who later changed his name to Anthony), a black or black-hispanic mixed man, and his future wife Mary, a black woman, were among the very first non-white and non-Native American people to arrive in America. They came by boat to Virginia in 1619 as indentured servants. Upon their release they were given land and eventually became wealthy enough to take on indentured servants of their own.
A black man by the name of John Casor became one of Anthony Johnson's indentured servants. Johnson failed to release John Casor after a time, so Casor ran away. Casor plead his case in court, but Johnson convinced the court that Casor had agreed to be Johnson's slave for life. The Virginia court ruled in Johnson's favor in 1654 or 1655, making John Casor America's first officially recognized slave.
In 1665 Anthony and Mary Johnson moved to Maryland to live out the rest of their lives, but not before setting the precedent for lifelong slavery of Christians. It didn't take long for rich land-owners and politicians to contrive ways to change the focus of slavery from religion to color, and slavery as we generally know it was born.
Sources:PBS: The Blurred Racial Lines Of Famous Families
Russell, Colored Freemen As Slave Owners In Virginia
Wikipedia Article: John Casor
Wikipedia Article: Anthony Johnson (American Colonial)