It would be very much appreciated if someone can provide some sources that permit a Jew to enter a church premises (not the actual sanctuary). I've signed up for a classical guitar festival that's in a church, but does not take place in the sanctuary, but rather other rooms within the church's building. This is an activity which would greatly benefit my career should I go, and it's not hosted by the church, but rather a separate organization. I've asked a Rabbi in person, who said yes, but I struggle to find sources. Additionally, would passing through the sanctuary to make it to another room be mutar? Thank you!
Based on my findings it should be prohibited.
The חכמת אדם פ"ד, ט"ז explains that because an Avodah Zara is assur behana'a (one is not allowed to have any enjoyment from it) one can not enter the house of an Avodah Zara and to look at its beauty. There is even a Mitzvah to not walk with 4 Amos of its entrance.
Because Christians believe in שיתוף they are considered ovdei avoda Zara.
Accordingly, Rabbi Ovadia ( יחווה דעת ד,מ"ה) rules that one is forbidden to enter a church.
For further reference see