Let me ask you a question: Do you apply this idea or principle to any other part of life? In other words, do you think it really doesn't make any difference how you act, as long as you are sincere?
For example, if the doctor writes you a prescription, do you tell him that you aren't going to bother getting it filled because it doesn't make any difference which medicine you take, as long as you're sincere? I seriously doubt it. Or do you avoid warning your children about the dangers of traffic, because you think it won't make any difference what they do? No, of course not. Life constantly confronts us with decisions - and every one has a wrong way and a right way.
The same is true with God - and one reason is because God Himself has given us the right way. Only one thing separates us from God, and that is our sin. But Jesus Christ came into the world to make our forgiveness possible - and He did this by becoming the final and complete sacrifice for our sins.
When we trust Christ and open our hearts to Him, God forgives us and gives us salvation as a free gift. The Bible says, "For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord" (Romans 6:23). Don't be deceived, but receive God's gift by committing your life to Christ today.