Unmarried parents

After 15 years of research I finally found my Gt Grandfather Anders Christen Christensen born in Vestre Aker, Oslo in 19th Dec 1839. He married Marie Nilsdotter[ Swedish] in 1869 in the Nes Church, Romericke before sailing to New Zealand in 1871 with first child Martha. Anders mother was Randine Pedersdatter, unmarried, and by a stroke of luck, the minister wrote on Anders baptism records in the Aker church, that his father was Christen Andersen, a Musketeer. Now what I would like to know - is how do I go about getting Christen Andersen's birth date when there is no marriage record to point you in the right direction. Or for that matter records pertaining to his army life. Help!