All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, you should know that it is not permissible for you to sell templates to forbidden sites, like "evangelical" sites, gambling sites or the like.
If we assume that you had sold them while you did not know about them, and then they ask you for technical support, then it is not permissible for you to provide technical support to forbidden websites, as this is helping in sin and transgression. You should clarify to them that you did not know about their reality and that if you had known you would not have sold to them.
Moreover, it is not permissible for you in the Religion of Allaah to provide support to such websites and that condition (providing support) is void as the Prophet said: "Whoever stipulates a condition that is not in the Book of Allaah, then that condition is void, even if it were one hundred conditions, as the condition of Allaah is more binding and more firm.” [Al-Bukhari and Muslim and its wording is according to Al-Bukhari]
Ibn ‘Umar or ‘Umar said: “Any condition that contradicts the Book of Allaah is void, even if someone stipulated one hundred conditions.” [Al-Bukhari reported it without its complete chain of narration]
Allaah Knows best.