必／会， 这个句子为什么用”必“？必 vs 会？ Why does this sentence use "必“？
必 emphasises and reinforces much more the idea of definitive, firmness.
我会帮助你 means I will help you
我必会帮助你 means I will definitely help you

You would say 我必会帮助你 when you are 100% sure you will commit to help the person. 
You feel much more secure about the other person's commitment if the person tell you 我必会帮助你. 

必 also emphasises on the idea of success as in when you say 我必会帮助你, you are also quite sure that you can help the person successfully.  

You could say 我一定会帮你 to mean I will definitely help you. But you feel less secure about if the person can actually help successfully. 

God is all-powerful, hence he can use 必. You feel secure and confident in his presence and in this question, you feel secure and confident from his words when he tells you that he will help you.