Thank you for reaching out with your prayer request. We understand that you're facing a spiritual battle for your fiancé, and we're here to pray and provide biblical guidance with you.
Firstly, it's important to acknowledge that spiritual warfare is real, and it sounds like your fiancé is experiencing this deeply. The Bible tells us, "For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places." (Ephesians 6:12, WEB).
It's concerning that he is swinging between extremes. One moment he proclaims to do anything for Jesus, and the next he denies the truth of the Bible and God's love. This could be a sign of spiritual rebellion and demonic influence. It's crucial to remember that God's word is clear about such influences. God does love us and wants the best for us. "For God so loved the world, that he gave his one and only Son, that whoever believes in him should not perish, but have eternal life." (John 3:16, WEB) He proved His love for us when He sent His Son, Jesus to die for our sins.
However, it is only through Jesus that we can be saved. The Bible tells us, "For there is one God, and one mediator between God and men, the man Christ Jesus," (1 Timothy 2:5, WEB). The last part of your prayer request was missing a reference to Jesus. Therefore, please remember it is only through the name of Jesus we have access to God.
Let's pray together for your fiancé:
Dear Heavenly Father,
We come before You in the name of Jesus, lifting up our ###'s fiancé. Lord, You know the battles he's facing, the whispers of the enemy that are causing him to doubt Your truth and Your love. We rebuke these attacks in Jesus' name, for You have not given us a spirit of fear, but of power, love, and self-control (2 Timothy 1:7, WEB).
We pray that You draw him close to You, that You open his eyes to the truth of Your word, and that You fill him with Your Holy Spirit. Help him to submit to You fully, Lord. He has been given many signs, prophetic words, and personal experiences to know God is real. Help him to have faith in these experiences. Help him to grow to the point of being a godly spouse with your help. We trust he will find his victory in You and You alone.
We also pray for his deliverance from any demonic influences, Lord. Cast out any spirits of rebellion, doubt, and self-pity, and replace them with Your peace, truth, and love. Help him to see the importance to submit to You.
In Jesus' name, we pray that God's will is done in his life. We pray that the spiritual battle he is experiencing will cease, and that he will walk steadfastly with You, Lord. We pray for his conviction of sin.
In Jesus' name, we pray. Amen.
You are both experiencing struggles with spiritual warfare. Continue to pray together, for each other, and for your future. Be alert and watchful with regards to your faith. Your enemy, the devil, is like a roaring lion, sneaking around to find someone to devour. (1 Peter 5:8, WEB).
We encourage you to continue standing strong in your faith and we pray for God's guidance and wisdom for you both.
Regarding your concerns about his YouTube use, remind him that we should be sober and self-controlled (1 Peter 4:7). If he is not using his time to glorify God, he should reevaluate how he is spending his time.
Lastly, we applaud your commitment to marrying a man of God. Remember, "Do not be yoked together with unbelievers. For what do righteousness and wickedness have in common? Or what fellowship can light have with darkness?" (2 Corinthians 6:14, WEB).
We will continue to pray for you both, trusting that God will hear our prayers and act according to His will.
Prayer Focus: God, Thank You for loving me. God, I ask You in Jesus’ name please bless me with everything that I stand in need of and everything You want me to have. 
