You can describe something as nonsectarian if it's not connected or affiliated with any particular religious or political belief.
A college is nonsectarian if it isn't associated with a religion or church, and a Sunday school class that teaches all religions is also nonsectarian. Something that's sectarian follows the rules of a particular group or sect, and it's the Latin word secta that's at the root of both sectarian and nonsectarian. The literal meaning of secta is "a way or a road," and its figurative meaning is "a following, school of thought, or religious group."
adj not restricted to one sect or school or party
ecumenic, ecumenical, oecumenic, oecumenical
concerned with promoting unity among churches or religions
occurring between or among or common to different churches or denominations
not restricted to a particular religious denomination
not bound or devoted to the promotion of a particular denomination