It is the responsibility of all historians to lay aside their biases and consider the evidence as objectively as possible. It is not the responsibility of the evidence to satisfy the biases of historians. (1)
~ Michael R. Licona
Galatians is a critical book for Christian casemakers. Even skeptics accept that it was written by Paul and that it was of the earliest New Testament writings.
Even more significantly, even if a skeptic doesn't accept it as the inspired word of God, it demonstrates that “Jesus’ followers started with a high Christology, or view of Jesus Christ. Such a viewpoint was not something added later. Paul and the early church saw Jesus as “Lord” (Gal 1:3), as the post-resurrection Revealer (1:12), as the preexistent One sent forth by the Father (4:4), and as the Changer of the world structure (6:14; cp. 2:20–21).” (2)
As a casemaker in a pre-evangelism setting you don’t even have to have all the answers. Maybe you remember little of the above argument, but you still do good work if you ask a question that you know is one stepping stone on the path to Truth.
Even by secular historical standards, many New Testament writings are among the best documents we have from the ancient near east. And when you ask, “Which New Testament books do even critical scholars accept?”, you leave them with the curious presupposition that critical scholars do accept many of the NT books as credible historical documents.
(1) This book is neither light nor short, but but it is a fabulous argument from the position of historical evidence. Spoiler alert: Resurrection? Yes!
Michael R. Licona, The Resurrection of Jesus: A New Historiographical Approach (Downers Grove, IL; Nottingham, England: IVP Academic; Apollos, 2010), 614.
(2) I couldn’t recommend the The Apologetics Study Bible more highly.
“Introduction to Galatians,” in The Apologetics Study Bible: Real Questions, Straight Answers, Stronger Faith, ed. Ted Cabal et al. (Nashville, TN: Holman Bible Publishers, 2007), 1751.