The answer: by faith alone.
Romans 3:21-22
21. But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets;
22. Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference:
2 Corinthians 5:21
21. For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him.