What is the ruling on entering churches for tourism? And what is the ruling on someone who does that if there is something forbidden within them?

It is not permissible to enter churches for tourism due to the presence of images and statues, and because they contain elements that contradict the religion of Islam. Indeed, Umar ibn al-Khattab refused to enter a church that had images, and Ibn Abbas would not pray in a synagogue that had statues. It is mentioned in the Hadith that the Prophet, peace and blessings be upon him, considered those who make images to be the worst of creation. Entering them is an affirmation of their falsehood, and whoever has entered them must repent and seek forgiveness.