Final answer:
The correct sentence is: Christians commonly believe that there is one God.
Step-by-step explanation:
The correct sentence is: Christians commonly believe that there is one God. The sentence is correctly capitalized with a capital C for Christians and a capital G for God. It does not contain unnecessary capitalization for non-proper nouns. In English, proper nouns, such as the name of a religion or a deity, are usually capitalized, while common nouns are not.
Learn more about Capitalization in English sentence