Responding to Lucas85's original post:
Paul's letters are in line with everything Jesus taught. If you believe otherwise, you lack a full understanding of the content contained in either the gospels or the epistles. A fuller study of both the gospels and epistles will show that they are in harmony with each other.
First, let's define some terms according to what they Biblically mean:
-What is the Kingdom of God? It is the the rulership of God in our life.
-What is Good? God is good. He defines good by who He is. Anything can only be said to be good insofar as it lines up with who God is.
-What is sin? Sin is rebellion against what God has said we must do.
-What is belief? It is not merely mental acknowledgement of something as true. The Greek word for belief implies that you live your life as though something is true, that you build and rest your life on that truth. There is action associated with it as proof that you really believe it.
Your first mistake is equating choice with effort.
We have a choice to obey God, but we cannot by any action of our own enter into Heaven.
We enter into Heaven by our choice to follow Jesus, who through His work has provided us the way to enter Heaven.
The idea that man cannot save themselves and enter Heaven by their own works, apart from the work of Jesus, is a concept we see in the Gospels and letters of the other disciples - It is not just a concept we find in Paul's letters.
John 3:13, John 8:21-24.
Nobody has ever ascended into Heaven, except Jesus. In fact, they can't on their own.
Mark 10:18
Nobody is good but God
Matthew 19:24-26
It is impossible for man to enter into the Kingdom of God and save themselves, but with God it is possible.
John 10:9, John 3:14-16
Jesus is the door to salvation.
John 8:24
Jesus specifically said we must believe in Him to be saved.
Without Jesus, we will not have eternal life, due to sin.
John 5:39-40, John 4:13-14.
You cannot find life apart from Jesus
John 10:11
John 3:14-16
Matthew 26:27-28
Jesus gave his life in a sacrifice for the forgiveness of thier sins, to bring salvation to His people. We may make the choice to follow Jesus, and we may make the choice to partake of what He offers us, but it is Jesus who leads us to salvation and provides the way. Nobody in history ever has been able, or will ever be able, to have what He offers by their own efforts. He is the sole source of salvation.
John 10:16, John 10:27
Those who are Jesus's people are those who obey Him (follow His voice).
John 8:24
There is something fundamentally different about people compared with Jesus.
John 3:5, 1 John 3:9.
Entering the Kingdom of God requires a spiritual rebirth.
Matthew 7:21
Entering the Kingdom of God requires doing the will of God.
Mark 9:47
To sin is to not be in the Kingdom of God.
Paul merely expounds and elaborates on these truths.
Your second mistake would be to think that Paul is preaching a message that puts no responsibility on the individual. That is not true.
1 Corinthians 5:1-5
Here we see Paul telling them to discipline a man who is in blatant rebellion to God, so that he might repent of his rebellion and have eternal salvation.
Neither Jesus, Paul, nor the other disciples taught you could live anyway you wanted, in rebellion to God, never making an choice to reform in any way, and be ok because you claimed that you believed Jesus died for your sins. As James said, even the demons believe, but faith without works is not real living faith (James 2).
Third; there's nothing in the text itself which would lead us to conclude your view of Paul is true, that he felt the need to invent a religion to feel better than others by being more exclusive. You cannot expect to find the truth by engaging in Eisegesis, inserting your own ideas into the Bible text. Instead, you must let the Bible text speak for itself (Which is Exegesis).