Paul's Epistle to the Galatians Edit
"Paul, an apostle, (not of men, neither by man, but by Jesus Christ, and God the Father, who raised him from the dead;) And all the brethren which are with me, unto the churches of Galatia: Grace be to you and peace from God the Father, and from our Lord Jesus Christ, Who gave himself for our sins, that he might deliver us from this present evil world, according to the will of God and our Father: To whom be glory for ever and ever. Amen." - Galatians 1:1-5 (KJV)
After Paul's successful evangelism work in Galatia, the church of had suffered the problem of Judaizers preaching that Christians were still bound to some Old Testament laws and practices, among others, circumcision and certain dietary restrictions. Judaizers argued that Paul was not a "real" apostle, claiming that in order to sugar-coat the gospel, he had removed legal requirements. Paul replies by reassuring his apostolic authority, and explains how legal requirements perverts the Gospel. This is where the fundamental of Christianity, sola fide is introduced.
It is in this book that the essential doctrine of justification by faith in Christ alone is best explained. See verse 2:16, "...a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified."