I thought it might better fit as a first or second sentence because a statement about the mission of the organization strikes me as more of a topic sentence for a paragraph than a supporting sentence It seemed to better fit the first paragraph as on my reading the second paragraph seems to summarize biographical information while the first paragraph defines the group and its reason for notability However I can see it as a topic sentence of the second paragraph with something like The SPLC describes its mission as fighting hate and bigotry and to seeking justice for the most vulnerable members of society  or maybe you can show a draft of the variant you had in mind