Let's say you know someone who has walked away from God, or has never known or believed in God. And you pray and pray for them. Can you have faith on their behalf?
Tricky one this. So let's analyse it carefully.
Yes, you can have faith on their behalf. This means you have faith in God that God loves them as one of His creations, and that He dearly desires that they return to Him and spend eternity with Him.
So, by having faith on their behalf, you are actually opening a channel through which God can respond to your prayers. God listens to you and is aware of your pleas on behalf of someone else. So your prayers are answered by God creating opportunities for that person to return to God.
But this bit is important: God creates opportunities for that individual to return to Him in response to your prayers. But God never controls that individual. He never forces that individual to return to Him. God merely shows him the way. But the final decision to return to God or not rests with that individual.
Thus it follows that yes ... you can have faith on behalf of someone else. Your faith, (sincere belief), is that God will answer your prayer by calling your friend back into the fold. Remember Christ saying that He would leave the ninety-nine sheep in search for the one that is lost? So, God will call back that lost sheep, but He will never force him back.
It is worth repeating: Through your faith you pray for another person. God hears your prayers and responds by allowing/creating opportunities for that person to return to God, or find God in their lives. God does not force that person in any way. To do so would be forcing His will, (and yours), on that person. The person must always be free to make his own decision. Your prayers and your faith are a channel and a plea to God on this person's behalf.