Is it permissible to provide headphones to Christians, knowing that this might help them hear the lessons of priests and monks better?

There needs to be a detailed explanation regarding the ruling on helping a Christian install a hearing aid. If the sole purpose is for him to listen to the priest's speech in the church, then it is not permissible, because it is assisting in sin and polytheism (shirk), and non-Muslims are accountable for that. However, if the hearing aid is installed to help him with his daily life matters and to treat his hearing loss, and the church is just one of the places where he benefits from it, then there is no harm in that. This is because the original purpose of installing the hearing aid is treatment, and a Muslim treating a non-Muslim is permissible, and even encouraged. The Christian alone bears the sin of polytheism. What is excused in subsidiary matters may not be excused otherwise. Thus, listening to sin does not alter the ruling of the original act, which is the permissibility of treatment. One should also seize the opportunity to invite him to Islam.