It is based on the based belief that in order for people to go to heaven, they have to be saved. This belief is stemmed from
Romans 10:9-13
New International Version (NIV)
“9 If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved. 10 For it is with your heart that you believe and are justified, and it is with your mouth that you profess your faith and are saved. 11 As Scripture says, “Anyone who believes in him will never be put to shame.”[a] 12 For there is no difference between Jew and Gentile—the same Lord is Lord of all and richly blesses all who call on him, 13 for, “Everyone who calls on the name of the Lord will be saved.””
They believe that those who aren’t saved are going to hell. This is a harsh thing to say but the reality is: there is a heaven and a hell. Not everyone is going to heaven nor hell. If you are Catholic, there is also purgatory to add into this equation. Theologians have discussed each over the centuries and it comes down to the idea: is Jesus your personal saviour and is God your God? If not are you going to heaven, it’s hard to know. Most in those denominations will say no, you aren’t. Many also like to make assumptions and judgements regarding who is going where and in the end, it’s God who makes those decisions. Not us, only God. I believe there is some grace inside this equation but we also need to also look at our own lives and ask that important question. It is something that is extremely important to consider. We just can’t take God for granted. Jesus didn’t die because it was the thing to do. He spread his blood so that all of us could find ever lasting life in his beloved father. If you get startled by that question, I suggest that you start praying to God and ask him for guidance in your life. Sometimes if we ask that question, it can start some personal reflections and help us grow closer to the Lord.
Some will be very forceful and use it as a scare tactic. I’ve seen it done that way but instead of getting angry, pray and ask God to show you anything in your life you that is preventing you from seeking his will.
These denominations have what they call the sinner’s prayer: which is a beautiful prayer that is for all Christians to pray. If prayed wholeheartedly, it can forever change a person’s spirit because it is affirming what they believe which is found inside the trinity. For example:
"Heavenly Father, have mercy on me, a sinner. I believe in you and that your word is true. I believe that Jesus Christ is the Son of the living God and that he died on the cross so that I may now have forgiveness for my sins and eternal life. I know that without you in my heart my life is meaningless.
I believe in my heart that you, Lord God, raised Him from the dead. Please Jesus forgive me, for every sin I have ever committed or done in my heart, please Lord Jesus forgive me and come into my heart as my personal Lord and Savior today. I need you to be my Father and my friend.
I give you my life and ask you to take full control from this moment on; I pray this in the name of Jesus Christ."
Amen.
I hope this answers your question.