1.the christians go to a church to pray.

2.christmas is celebrated on twetififth december.

Proper nouns ( names of specific things) are always capitalized. The first word in a sentence is always capitalized.
http://owl.english.purdue.edu/owl/resource/592/01/
This site will give you all the rules.

The second sentence needs to read "on the twenty-fifth of "
I hope this helps.

is this sentance correct_
She also has a wonderful relationship with her husband, but for some reason she decides to escape from her home.

Yes, the sentence you provided is grammatically correct. It has proper capitalization and punctuation. The use of "but" in the sentence indicates a contrast between the wonderful relationship with her husband and her decision to escape from her home.