Is it permissible to pray in a church, and is it permissible for a hired worker to work in it to decorate it, and is the one who does so sinful?

There is no harm in praying in a church if one needs to, and does not make it a habit. This was permitted by Al-Hasan (definition), Umar ibn Abd al-Aziz, and Al-Sha'bi, based on the generality of the Prophet's saying: "Wherever prayer overtakes you, pray, for it is a mosque." However, some scholars disliked praying in them due to the presence of images.

As for working in a church, such as building or decorating it, this is not permissible because it assists them in their falsehood, and because its earnings are not lawful. Allah says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression."