Lord Jesus Christ, thank you for your love for me. I admit that I have gone my own way and I am sorry for hurting you and others. Please forgive me. I now turn from things which I know are wrong. Thank you for dying on the cross for me. Thank you for offering me forgivness and a new start. I now gratefully receive your gift. Please come into my life, be my Lord and my Saviour and live in me by the power of your Spirit. Come to be with me for ever. Amen.
If I have done the above steps then I can be sure because of:
"Anyone who comes to me I will in no wise cast out." (John 6:37). Find your own promise in the New Testament to live by. I can be sure I'm a Christian, not by relying on my feelings, which go up and down, but by relying on God's promise.
I can never be good enough to achieve heaven. But God has not abandoned me to my plight. He sent his only Son to die on the cross to reconcile me to him. All I have to do is to accept his sacrifice on my behalf. To say "I am a Christian" is to be faithful to the debt I owe him - it is not boasting about me. Being sure I am a Christian is relying not on my achievements but on his sacrifice.
"God has put his seal on us, and given us his Holy Spirit as a guarantee" (Ephesians 1:13). Things actually happen inside the Christian which (when we remember them) show us that God is changing us. The most important thing of all is that in our hearts we become more and more convinced that we really do belong to God's family. "When we cry Abba, Father, it is the Spirit himself, bearing witness with our spirit that we are children of God" (Romans 8:15).