Question
I am non-Muslim person and very interested in Islam. Am I allowed to enter the mosque or am I unclean?

Answer

The physical bodies of non-Muslims are not filth [najis].

Evidence from primary texts

It was reported that the Prophet (peace and blessings be upon him) received the delegation of Bani Thaqeef, who were polytheists, in his mosque. If their physical bodies had been deemed filth, he would not have hosted them inside the mosque.

It was reported that during the year of Hudaibeya, Abu Sufyan (may Allah be pleased with him) entered the mosque, though he had not yet embraced Islam.

On the day of the conquest of Mecca, the Prophet (peace and blessings be upon him) said, "Whoever enters the mosque will be safe."

The Messenger of Allah (peace and blessings be upon him) dispatched a military expedition to the area near Najd territory. They captured a person called Thumama ibn Athal and tied him to one of the mosque's columns. [Bukhari]

Allah the Almighty allowed Muslim men to espouse women from the people of the Book (Ahl al-Kitab). Allah did not make it obligatory for Muslim men to wash themselves if they are inevitably touched by the sweat of their non-Muslim wives.

The metaphorical meaning of the words of Allah "Indeed the polytheists are filth" (Qur'an 9:28) refers to the corruption of their beliefs and not to the filthiness of their bodies.