Which book of the Bible teaches the most salvation?
In my opinion, Ephesians may be the most jampacked book containing teaching about salvation. For its six chapters, it speaks about salvation per capita more than anywhere else. The book opens with the blessings we receive in our salvation with Christ (Ephesians 1:3-14).
In Paul’s following prayer, he talks about knowing that God has called us to our inheritance, and believing according to the working of his might (Ephesians 1:18-19). Ephesians 2:1-10 explains that we are saved by grace through faith (Ephesians 2:6, 8).
Paul describes the difference in us in the former life that we had without Christ and the life we now have in him (Ephesians 2:1-4). He describes that we have become dead to sin and alive in Christ (Ephesians 2:1, 5).
We were raised up with Christ and seated with him in heavenly places at the moment of our salvation (Ephesians 2:7). This was only the work of God that we are saved, not our own works to save ourselves (Ephesians 2:9). And we are now the workmanship of God (Ephesians 2:10).
The end of Ephesians 2 focuses on the division between Jews and Gentiles before we come to Christ (Ephesians 2:11-22). Only in Christ have we become unified and part of his body. The old way of division and hostility toward God is now our way of peace in Christ.
In Ephesians 3, Paul explains that salvation is a mystery that God hid from people until Christ came (Ephesians 3:1-12). As we continue to read through the letter to the Ephesians, Paul now applies our salvation to our sanctification. Now that we have come to know Christ, we must grow in him (Ephesians 4-6).
For a small as the six chapter letter is, it packs a punch about salvation and explains it extremely well for Christians. Paul makes it clear how and why we are saved, and then what God expects of us. Many of the books of the Bible explain salvation in some way or another, but per capita I believe Ephesians explains her salvation the most.