If the strange looks you get from your fellow countrymen are enough to grate on you when the topic of atheism/religion pops up, it will be even more so here. In the USA, I never had anyone give me a second glance when it came up with me. Here, Japanese automatically assume most foreigners (and certainly all Americans) are Christians. Tell them otherwise, and you not only disappoint them, but you baffle them and open the door for a plethora of questions. Case by case on who really pursues them, but that's the reality.
Image. I meant DOING things, not how you appear. For example, feeling obligated to go to the company Christmas party because, well, that's what is expected of you. Feeling obligated to come to work dressed properly. BEING obligated to shut up and not complain about many situations you don't like (at work, especially).
Image can also include such things as having things, even the simplest store purchase, wrapped in 3 layers of paper and bags, being shouted at "Irashaimase!" as a greeting whenever you walk into a store (and shouted at as a thank you whenever you pass a store clerk and when you leave), etc. Some things about image are good, some are pointless but still done. Give a present at a wedding (or funeral), and you might be obligated to give far more than you would expect. Being obligated to RETURN a gift with a gift! There are so many things that involve image and not losing face.
Another is the difference between "honne" and "tatemae". Look it up.
I also really like the food mmmm eel and salmon and tempura. I love to go on Youtube and watch their TV and most of all I love the language. I've heard a lot of different languages and Japanese has got to be the most unique due to it SOV style and it straight forward communication. The Japanese language not only very pleasing to study but also very calm and easy to listen to. Sorry I like to rant on about the sound of it. hmmmmm, Technology and video gaming plays a huge roll in my life, and I love the Playstation 3, my car is Japanese and most of everything I use right down to my guitar.
Enjoy your homestay. How old are you? Be very polite with the homestay family, and heed their rules. Manners may prohibit them from telling you that you have broken the house rules, so be on your best behavior and think of the HOSTS before your liberal self.
As for Japanese language being "easy to listen to" and "calm", I suspect you have not heard a lot of high-pitched J girls screaming KAWAII! at cute things, or giggling hysterically even at 20 years of age. Or perhaps not seen real J TV enough to despise the incessant shouting on most programs by hosts and "talento" as a means to be funny. Watch and learn.
As for "straightforward conversation", you really DO have a lot to learn! Japanese conversation is the most indirect form of communication I know. I can't help but repeat myself by advising you to look up honne and tatemae.