I have not really had that type of experience but i remember that when I got born-agin myself there were dooubts in my mind about my being saved and for quite a while there were doubts about wether there was a God. This was something I had to battle and now that I look back I see the scripture that Jesus says all whom the Father has given me no one can take from me, but I will raise them up on the last day became true in my life.At a time I resorted to singing christian songs all the time wherever I was to give me a sense of who I had now become. I resorted to meditating and thinking on God all the time. You see I had come out of dire circumstances and I badly wanted to keep my faith, I did not want to return to the suffering I ad come out of so these things were very important to me. The devil always tried to put doubts about the existence of God in my heart but God always countered that in that I understood the wisdom and saving power of God. I was about 22 years at the time.
Your brother is fifteen and maybe has not the will power and experience or insight that I had at the time and may find it difficult to bring every thought captive. So you will help Him, you will pray and pray and pray and uphold Him before the Lord. You may share with him whatever God lays on your heart. I am confident that the Lord will help you because He seeks to keep your brother. This is no small matter and should not be taken lightly. When you see the fruits of this effort your faith in God will increase.
So go ahead and take every thought captive in prayer for you brother the Lord will grant you success. Forget the strong musical background forget everything you can see that appears to negate what God will do and is doing and simply believe that the battle is the Lord's and He has not lost any battle.
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CrownofGlory