We understand that you're going through a challenging situation and we're here to pray with you and provide biblical guidance. First, we commend you for valuing chastity and purity, which are biblical principles that please God. It's also encouraging to hear that the man who is proposing to you is now a believer.
Let's address the issues raised and turn to the scriptures for guidance. Firstly, it's important to remember that God can forgive and transform any individual, no matter their past. As it says in Isaiah 1:18 WEB, "Though your sins are like scarlet, they shall be as white as snow. Though they are red like crimson, they shall be as wool." The man who proposed to you has confessed his past sins and is now a believer, which shows that the Holy Spirit is working in his life.
However, your concerns are valid. It's normal to feel anxious about his past, but we must remember that God tells us not to be anxious about anything, but in every situation, by prayer and petition, with thanksgiving, to present our requests to God. In Philippians 4:6-7 WEB, it says "In nothing be anxious, but in everything, by prayer and petition with thanksgiving, let your requests be made known to God. And the peace of God, which surpasses all understanding, will guard your hearts and your thoughts in Christ Jesus."
Regarding your fears about Satan using his past to ruin your marriage, remember that God is the author of marriage and He wants it to succeed. In Matthew 19:6 WEB, Jesus says, "So that they are no more two, but one flesh. What therefore God has joined together, don't let man tear apart." Also, in Ephesians 6:16 WEB, it talks about the shield of faith, with which you will be able to quench all the fiery darts of the evil one.
Now, let's pray together:
Dear Heavenly Father,
We come before You in the precious name of Jesus, lifting up our sister who is seeking Your will regarding marriage. Lord, we ask that You grant her discernment and peace according to Your will. Help her to forgive his past sins, just as You have forgiven us all.
If this man is truly a changed believer and Your will for her husband, we pray that You give her a supernatural love for him, that surpasses his past. Help her to see him as a new creation in Christ, with his old self gone and his new self here. Father help her through any doubts or fears she may have and surround her with peace.
If he is not the one You have for her, we ask that You clearly reveal this to her and give her the strength to walk away.
In all things Lord, we ask that You work for the good of those who love You, who have been called according to Your purpose. And help her to always remember and find comfort in Jesus Christ. As it says in John 14:6, Jesus said to him, "I am the way, the truth, and the life. No one comes to the Father, except through me."
In Jesus' name, we pray. Amen.
Lastly, we encourage you to seek godly counsel from your pastor or spiritual mentors. They can provide you with additional guidance and support during this time. Keep praying and trusting in God's perfect will for your life.
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. 
