Interdenominational and Undenominational

Interdenominational adjective - Occurring between or among or common to different churches or denominations.

Undenominational and interdenominational are semantically related in non-denominational topic. In some cases you can use "Undenominational" instead an adjective "Interdenominational", when it comes to topics like nondenominational.

Undenominational adjective - Not bound or devoted to the promotion of a particular denomination.

Interdenominational and undenominational are semantically related in non-denominational topic. You can use "Interdenominational" instead an adjective "Undenominational", if it concerns topics such as nondenominational.