Acts was not about the birth of the church as much as it was about the fall of Israel culminated by the stoning of Steven in chapter 7. This book serves as a transition on how God is dealing with mankind as Jesus Christ returns to heaven and the apostles continue where Christ left off in his offering of the kingdom to Israel. As the 12 apostles focused on Israel, the risen, glorified Lord Jesus Christ did something unplanned, un-prophesied and unexpected when he revealed a secret to the apostle Paul ushering in a new dispensing of grace (chapters 9 through 28)....
Paul's letters include Romans through Philemon with many attributing his authorship to the book of Hebrews as well. However, Paul was the apostle of the Gentiles and given his revelation by the risen, glorified Lord Jesus Christ. Although grace and peace came by Jesus, the glorfied Lord Christ appeared to Paul and gave him the ministry of reconcilation (2 Corinthians 5:18-21); a ministry to the nations yet distinct from the 12 apostles. It has been the doctrine of the Church, the Body of Christ since his divine appointment by Christ Himself (Galatians 1:12). These letters will be in force as God's primary instructions until "the gathering" of believers unto Christ (1 Thessalonians 4:16-18, 2 Thessalonians 2:1)
After the "catching away" of the saints that comprise The Body of Christ to meet the Lord in the air, (1 Thessalomians 4:16-18, 1 Corinthians 15:50-58, 2 Thessalonians 2:1-3), the Tribulation Saints that get saved afterwards, will need books of the bible, those epsitles or letters to help them navigate through the dreaded Great Tribulation! In particular, the book of Revelation will be a blow-by-blow roadmap of what will occur so that the saints can escape the AntiChrist.