I Hate My Mother In Law Forum & Chat Board | MEAN FUTURE FATHER IN LAW - HELP

Hi all,
I am very new to this and just desperate to get opinions and help from people outside of my situation. My fiancee's whole family is very difficult in general. They do not have many friends or get along with people well, they keep to themselves. His mother was extremely obese and was very depressed and had suicidal tendencies in the past. She had gastric bypass surgery and is now thinner and more confident now. I used to think she was the issue in the family, but she has been very welcoming over the last couple of years.
Me and my fiancee have been together for 8 years.. I am 23 years old. Our wedding date is in a year. I let his mother be flower girl because it was her life long dream. She is extremely smart and intellectual, she got straight A's all through college. She had a stint with alcohol last year and ended up in the hospital for being found passed out outside of a walgreens. She has always appeared to be a perfectionist, but getting to know her and her life, I found out it was far from perfect.

His dad is 60 and is a very quiet man on first meeting. He doesn't smile or do the social norms that most people would do. For example, the first few times my mom said hi to him, he never responded back to her and he doesn't smile in pictures. He is from England and was adopted and never tried to find his biological parents. He is very in shape and soft spoken most of the time.
The deal with him and I has been that we have been mostly civil. He is extreme atheist and I am an exploitative Christian. My fiancee is agnostic. His dad has called my fiancee fat since about 5 years ago to his face. When my future SIL was 12, she accidently kicked his foot under the table and he called her a stupid b*tch (she is 19 now), and she cried in her room with no one to apologize or comfort her. His father has never told my fiancee that he loves him and when he was around 16 he told my fiancee real men do not hug when my fiancee tried to hug him. On past trips he has mocked me for my clothes, suitcase, and drinking bottled water. He speaks horribly about Christianity and God etc a lot, but I generally "tune him out," like my fiancee has told me to do and like my fiancee has done. But some stuff just gets under your skin and never leaves. There have been a few times where he has seemed up beat or positive to me, once was when he was drinking. His mom and dad mock us and say we are irresponsible for our "lifestyle" which basically means we eat out more than once ever 4 months at places nicer than the Olive Garden. My fiancee says that he feels he never really had a dad, and he has stood up to him for me a couple of times. We rarely have to see them, but as our "wedding present" they have offered to either have us come on a trip to Italy with them for 3 weeks, or give us that money to go on our own honeymoon for a week after our wedding (I only get 2 weeks off school then). This offer came after they had already told us a year ago we were invited to Italy.
Anyway, on Christmas (yesterday), I visited their house for the first time in a little while. Everything was fine in the morning, we had breakfast there. I went to visit my grandfather who is dying in a hospice and left my fiancee with his family. I came back and the subject of religion was brought up by my future SIL about someone's FB status. The person on FB had said it was Isaac Newton's birthday and she said "But more importantly it's Jesus' birthday!" His dad was nearby in the kitchen and he said "Jesus never existed." and my fiancee said back to him "Actually historically there was a Jesus.." and his dad came up to the table where the 3 of us were sitting and said "NO, there is NO SUCH THING AS JESUS, it is FAKE." His dad knows that I believe in God and am Christian. Something about what he said hit a switch with me and really upset me. I was fine and we left shortly after but then I broke down in the car. It hurts me that he could be so DISRESPECTFUL to my religious beliefs or any religious beliefs. I have never mocked or judged their beliefs that there is no God and never would. I have never said one negative word to the man. And when he said that to me, he looked right at me (I was the only one in the room who truly believes in a higher power), and it was Christmas.
Anyway, it made me decide that I cannot go on long vacations with this man. He mocks me, my beliefs, my eating and drinking water from a bottle habits, how much I go to the bathroom etc etc. My fiancee is behind me in not going on this 3 week vacation, but he has told me to keep sleeping on it. I don't feel like I want to subject myself to his father for 3 weeks, even though it would be a once in a life time trip. We went to Spain with them when I was 19 and it was really great, but the only amazing memories I have are when my boyfriend and I were alone together. On the trip, I was mocked multiple times by him for speaking about how spiritual the churches were to me, for not knowing my history, and for drinking bottled rather than tap water. At 24 I really don't want to be bullied by a bitter 60 year old man. I feel so guilty letting my fiancee down though and his mom. My fiancee has issues with his dad but he loves his mom and I know me not going will hurt them both a little bit inside. But what else can I do?