Is it permissible for the kaafir to enter the mosque to listen to a lecture?
Question

                Is it permissible for the kaafir to enter the mosque to attend a lecture or a lesson in which there is an invitation to Islam?.
Praise be to Allah.
    Yes, this is permissible, if we are confident that this 
    kaafir will not disrespect the mosque, because he is entering for a reason, 
    and not to harm or damage the mosque. Similarly it is permissible for a 
    kaafir to enter and stay in the mosque to repair something in the mosque, 
    because this is in the interests of the mosque, so it is also permissible 
    for him to enter to listen to lectures which may be the cause of his being 
    guided. The Prophet  (peace and blessings of Allaah be upon him) tied up 
    Thumaamah ibn Athaal in the mosque [when he was a prisoner of war, before he 
    became Muslim]. End quote. 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him).