Originally Posted by Wikipedia

Worship

As with all other Bahá'í Houses of Worship, the Lotus Temple is open to all regardless of religion, or any other distinction, as emphasized in Bahá'í texts. The Bahá'í laws emphasize that the spirit of the House of Worship be that it is a gathering place where people of all religions may worship God without denominational restrictions.
The Bahá'í laws also stipulate that only the holy scriptures of the Bahá'í Faith and other religions can be read or chanted inside in any language; while readings and prayers can be set to music by choirs, no musical instruments can be played inside. Furthermore no sermons can be delivered, and there can be no ritualistic ceremonies practiced.