What is the Shar'i ruling regarding building a mosque and a جامع (congregational mosque) for Muslims in Denmark that combines Sunnis and Shias, and what are the implications of that for prayer, the khutbah (sermon), and matters of worship? And is it advisable to continue with this project or to abandon it?

"Calling to Allah and establishing a center for it, especially in lands where Muslims do not find a place for prayer and da'wah, is of utmost importance. This is based on the Almighty's saying: [And who is better in speech than one who invites to Allah and does righteousness and says, "Indeed, I am of the Muslims"] (Fussilat: 33), and the Prophet's (peace be upon him) saying: 'For Allah to guide one man through you is better for you than red camels,' and 'Whoever builds a mosque for Allah, even if it be like a bird's nest for its eggs, Allah will build for him a house in Paradise.' It is advisable to continue uniting the Muslims so they agree to pray in the mosque and participate in the center, with the Imam adhering to the Sunnah."