The phrase "overriding purpose" means the main or most important reason behind something or the ultimate goal that takes priority over everything else. Full definition
The one overriding purpose of a liberation theology is to eliminate oppression. (religion-online.org)
Cobb, by overriding this purpose has taken on the worldview of the oppressor. (religion-online.org)
Cameron suggested that the central purpose of the EU «has been achieved» — to secure peace — and «today the main, overriding purpose of the European Union is... not to win peace, but to secure prosperity». (blog.politics.ox.ac.uk)