Contrary to popular belief, Christianity does not inoculate you from the effects of sin in your life.
The popular message that many preachers and Christians are spewing from their mouths is that "A free one-time prayer is the ticket to Heaven." Now, let's rewind and fill in all the gaps within this false advertisement:
First of all, the way to Salvation is Confessing with your mouth and believing in your heart that Jesus Christ is Lord, the concept of "free" is only made possible because Jesus Christ died on the cross and the blood He shed paid for the sins of the entire world, and furthermore there's nothing one time about it; being a Christian is a lifestyle and a life long commitment in which you are to live every single day for the glory of God... so let me clarify, that means every thought you think, every word you speak, and every action you make should have a motivation of glorifying God.
Don't get me wrong, I'm not saying you have to be perfect if that were the case we would all be hopeless because it's impossible, but once you've taken on this lifestyle and you are living for God your desire should be for your life to be a reflection of the perfect Savior that you were made in the image of, so you strive for that perfection daily.
“Not everyone who calls out to me, ‘Lord! Lord!’ will enter the Kingdom of Heaven. Only those who actually do the will of my Father in heaven will enter." Matthew 7:21
I have said all of that to say this, a one and done prayer is not an all-expenses paid trip to Heaven. Only the followers of Jesus that commit themselves to living a righteous life will see the Kingdom of Heaven. Saying that prayer and then turning around and stepping back into a hell filled lifestyle will land you in the very place that is a product of that sin, and from what the Bible says Hell is not a very pleasant place.
"If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, but only a fearful expectation of judgment and of raging fire that will consume the enemies of God." Hebrews 10:26,27