Jesus is the Son of God who became a man to be our Savior. He lived a perfect life, and he died a sacrificial death, so that when we repent, turning from our sins, and believe, putting our whole trust in Jesus for our salvation, he forgives our sins and gives us new, eternal life. That’s the gospel. We love the gospel.
James comes along and says if that’s your experience, if you believe in the gospel, if you have a genuine faith in Jesus so you are born again, meaning you have new life in Christ, then you are going to live a new way. If Jesus has saved you, you’re not going to be able to stop the changes he will make in you. If your faith is genuine, Authentic, more and more you will behave like you believe, trusting and obeying the commandments of Christ.