First, we are all blind to our own sin until we allow God to reveal our own hearts to ourselves. We don't see our sin, but we can see the sin in others. Think about how easy it is to spot pride in someone's life, yet, we, too easily have our own pride to deal with, yet, we overlook it because we either don't see it as really that bad or serious, or, we don't think we have pride, even though it is definitely there.
Secondly, Pelagians do not believe people are born with a sinful nature. They think Reformed people are causing people that hear the Gospel to be irresponsible if they were to teach that. In other words, the power of sin is totally under our control. We can choose to be sinners or not. But that is not what the Bible teaches.
In Genesis 3, we see the first human beings God created, turning from good to evil. They chose to do evil when they listened to the voice of the devil instead of God's voice. The world has been under a curse ever since that time. People would now be born into a spiritually dark world. They would be ensnared by sin. They will now become slaves to sin and be imprisoned by sin. Sin is no respecter of persons. Everyone has their fair share and then some. When we are born, we are already subject to the kingdom of Darkness. We don't choose to be in there. That is our default.
Third, Pelagians do not give room for spiritual growth or the process of sanctification. If the Pelagian doctrine is correct, then there is no sanctification process for we become perfected at salvation. But we have to be careful because we could lose this standing if we ever choose to sin. In fact, I am sure they would believe that if we choose to sin, then there is no more repentance available for that person. Once a true believer in Jesus Christ sins, it's over for him or her. (see Hebrews 6 and 10).
Finally, Pelagians do not appear to know anything about the love of God. They always focus on God's wrath and anger, and make it appear as if God is just waiting to destroy us. This passage from Isaiah shows something different though.
"Therefore the Lord waits to be gracious to you,and therefore he exalts himself to show mercy to you.
For the Lord is a God of justice;
One time, a rich, young ruler came up to Jesus and asked Him, "What must I do to inherit eternal life?' to which Jesus replied 'You know the Commandments." Then Jesus named some of the Commandments. The rich, young ruler responded by saying that he had kept these Commandments from his youth. So, Jesus asked the rich young ruler to do something else. This revealed what was in his heart. Jesus asked him to sell everything he had and then give the money from that to the poor. The rich young ruler was not happy about this question and he went away very sad. What had happened was that this man thought he was a good person without sin. He thought that he kept the Commandments, until Jesus challenged him with a question. Then, when he realized he really was a sinner, he was sad. But his sadness was not over his sin. It was over the fact that he thought he was a good person, but now Jesus showed him that he really was not a good person.
I think this is how the Pelagians think about themselves. They really do think that they are not sinners. They may look at the sin in their past lives and believe that Jesus died for their sins, but now it is up to them to 'stop sinning' and if they don't, they will become toast.
How much 'Pelagianism' is in our own thinking? We need to pull it out if it is there. We also need to be patient with other believers, especially those who are weak. The doctrine of Pelagianism could destroy many. It could also be very discouraging and is certainly deceptive, for none of us can stand up against the power of sin in our own strength. It takes the power of God, and the blood of Christ to set us free from sin. No amount of human willpower will take away our sin, nor will it take the desire for sinning away. That can only come with a new heart.
When we come to Jesus in repentance and faith, we receive the forgiveness of sins. But there is so much more! We are also set free from sin. We can be delivered from sin and its snares. We have a new heart. Now we can obey God and grow in our love and affection for God and others. This is not something we can do on our own. Only God has the power to break sin in our lives.