وَ  لَا   تُصَلِّ  عَلٰۤی  اَحَدٍ   مِّنۡہُمۡ   مَّاتَ   اَبَدًا   وَّ  لَا  تَقُمۡ   عَلٰی   قَبۡرِ ہٖ  ؕ

And never offer Ṣalāĥ for any of their dead, and never stand                  by his grave.

[Kanz-ul-Īmān (Translation of Quran)] (Part 10, Sūraĥ At-Taubaĥ, verse 84)

However, if those offering the funeral Ṣalāĥ were unaware of his conversion to Christianity, (etc.) and were considering him to be a Muslim according to what they knew, and they remained unaware till his funeral/burial, they will not be objected to since they assumed that he was a Muslim, and as such these acts were obligatory for them. But if they were aware of his Christianity, (etc.) and they still performed his Ṣalāĥ and burial, then they committed an absolutely grave sin. As long as they do not repent of this sin, Ṣalāĥ in their Imāmat is invalid; however they will not be treated as apostates as they will not become disbeliever s on committing that sin.

Our sacred Sharī’aĥ (Islamic Sacred Law) defines a straight path; it does not like immoderation in any affair. However, if it is proven that, despite being aware of his Christianity, (etc.), they did so not only due to their ignorance or some worldly interest but also considering his Christianity, (etc.) worthy of respect and deserving of funeral and burial, all those having this intention will, no doubt, become apostates and disbelievers. It will be Wājib for the Muslims to consider them apostates in all