There is only one lawgiver and judge, he who is able to save and to destroy. But who are you to judge your neighbor? (ESV)
I think there is a reason why television shows that are all about judges and juries are very popular. We get into the whole story of the criminal, just waiting for him to be caught. We have, in fact, made up our minds about the guilt or innocence of the person in the show. We imagine ourselves in the position of the judge, declaring the words, "I find you guilty."
We seem prone to declaring guilt in people's lives because it makes us feel like we are better than the one accused of guilt. James is addressing that very flaw in us. He reminds us that there is one lawgiver and judge, and we are not it. God himself is the one who sets the standard that we are all to live by. He alone is the one who can judge.
Imagine a scenario in your church. A young woman becomes pregnant out of wedlock. There will be some who will want to declare her guilty of sin and find some way to shame her. We are not the judge and jury. What should we do? Encourage the young woman. Offer to help her raise the child to know Christ. Assist her with her financial needs in caring for the new child.
We can certainly look at Scripture and know how God would judge. The issue is that we should allow God to have his role as judge, recognizing our role in offering grace. We are all going to stumble and fall. We need to see the stumbling in our brother or sister as an opportunity to demonstrate the grace and mercy of God, not an occasion to be a judge. Take to heart these words from Romans:
 Or do you presume on the riches of his kindness and forbearance and patience, not knowing that God's kindness is meant to lead you to repentance? (ESV)
Be an agent of God's kindness, not his wrath.