Believing that God cannot help or save you is a serious sin. Denying that Christ is not God is perhaps an even more serious transgression. There are many good people who are law-abiding, never try to hurt anyone, and even love to help others in need; but they knowingly do not believe in the God of Christ.
John 14.6: Jesus said to Thomas, "I am the way and the truth and the life. No one comes to the Father except through me." It shows that we have no way of bypassing Jesus and must inevitably go through him. In Matthew 10.33, Jesus said to the apostles, "Whoever denies me before others, I will deny before the heavenly Father." It shows further that you can be a good person and still be denied by Christ in the end. The correct order of business for us is to accept and believe in Christ and then be good for his sake.