Is the fact that the bodies of some Christian priests do not decompose after their death, but rather appear in good condition without embalming, considered a miracle from God and a sign of the truthfulness of their methodology, especially since Christians consider this as proof of their truthfulness?

The phenomenon of bodies not decomposing is exclusive to prophets, as Allah has forbidden the earth from consuming their bodies. As for Muslims in general, the non-decomposition of some deceased bodies is not considered proof of the validity of the deceased's school of thought or religion. Rather, it is a miracle (karamah) for some of Allah's saints and mujahideen, and it occurs in their natural graves without chemical or environmental intervention.