If Some Day...
 

If some day you see me knelt down, it will be for raising up someone who fell down.

Could you please check my sentence?

Re: If Some Day...
 

... it will be to raise up someone...

to + infinitive describes purpose - ie why you do something. Other examples :
I'm going out to do some shopping.
I phoned John to tell him about the meeting.

for + gerund = use - ie what something is used for.
Don't use that liquid - it's for cleaning the floor, not for washing dishes

The rest is OK grammatically, but sounds a bit stilted. It's difficult to comment without knowing the context, but I'd probably say something like : If ever you see me kneeling down, it will be because I'm helping someone else get up.