To properly understand Paul’s term, justification by faith, it’s helpful to consider the historical religious context in which Paul wrote. A principle of personal works under the Law of Moses had operated in Jewish history for about a millennia and a half prior to Christianity. But when Jesus came, a massive shift on many levels took place. He introduced the principle of faith in Him. He became the author of a New Covenant and even made it available to the entire world. Those who became believers of Christ accepted His terms of salvation rather than depending on their own merits under the Law. The Jews, who were long accustomed to the works-based principle of righteousness, would naturally have difficulty with the sudden and dramatic change brought about in Christ. Of course, God knew that transition to Christianity could pose a difficulty for His people. It was an expression of His mercy that He directed Moses to foretell of such a change involving One who was to come, that is, Christ (Deuteronomy 18:15-19). The prophet Jeremiah, likewise, foretold the change of covenant that would take place (Jeremiah 31:31-34), along with others. These prophecies should have provided the Jews with a basis for accepting the new faith principle of salvation (Romans 1:2). While they believed surely in the coming Messiah, they limited their focus on prophecy regarding Him restoring their place in the world (Acts 13:27). Consequently, their limited acceptance of prophecy led them to oppose articles of the new faith-based covenant, especially regarding the discontinued necessity of the rite of circumcision, which had been a signature feature of Judaism.
As mentioned, the principle of salvation in the New Testament is faith in Christ (John 3:16; Acts 16:31; Galatians 3:23-25; Ephesians 2:8-9). That is not the plan of salvation but it is the principle of salvation. What it means is that, if we truly have faith in Christ, we will do what He prescribed for salvation instead of trying to save ourselves some other way. From a Pauline perspective, the theological expression, justification by faith, stresses this principle of faith in Christ for salvation as opposed to the works of the Law.
Acts 2:38 is the plan of salvation because the tenets expressed therein were mandated by Christ in His Great Commission. The Great Commission is a mandate to all who believe in Jesus. Therefore, the tenets of salvation that are contained in it are mandated by Christ for use in evangelism and are the terms for salvation. Christian evangelism and salvation began on the Day of Pentecost. Peter spoke as a commissioned minister and preached the plan of salvation that every person who hears the Gospel is to obey and every New Testament minister, likewise, is commissioned to preach. Acts 2:38 was accepted by all the Apostles and became the orthodox rule of faith for salvation (Acts 2:42). And three thousand souls were saved that very day.
The tenets of salvation revealed on the Day of Pentecost for the Christian era are the faith response of all who believe in Christ. Because salvation occurs as a result of faith, it came to be abbreviated in speech and theology as, faith, and as, justification by faith. The futility of the former covenant of works was reinforced by Paul’s insistence that no one could be justified by the Law (Galatians 3:11). In the New Covenant, justification is solely by faith in Christ who is the Author of eternal salvation (Hebrews 5:9). After conquering death, hell, and the grave, Jesus made known to His Apostles the tenets of faith in the Great Commission, which included baptism in Jesus’ name for the remission of sins. Remission of sins is, of course, the essence of justification. The Apostles faithfully preached this message, beginning on the Day of Pentecost. And it is the message to which we also are bound. Having faith in Christ necessarily includes adhering to all He taught. Nothing less could be deemed faith.
It’s remarkable how Paul’s theology of justification by faith came to be so misunderstood. He did not intend to undermine the salvation of Acts. God forbid. That would be the furthest thought from his mind. He was saved by Acts 2:38 and preached it. The salvation of Acts is that of faith. Every part of Acts 2:38 is faith in Christ and obedience to these tenets comes solely through faith in Him. Paul certainly never dreamed that his words would be so misconstrued, as has happened in the last 500 years, so as to mean mere faith in Christ apart from the tenets of salvation, which are prevalent in his own experience and, indeed, in all the Gospels, Acts, and Epistles.
Conclusion
Paul was contrasting this principle of faith with what was interfering with it on the part of some Hebrew Christians, namely, the works of the Law. Justification came by what Christ had instituted, namely, baptism, which is expressly revealed to be for the remission of sins (Acts 2:38; 22:16), not by what had preceded it, that is, the works of the Law. So the controversy that existed in early Christianity was between the faith that Christ had established and the works of the Law, which had previously existed. Summarily, it could be said that it was between Christ and Moses (John 1:17). But never did Paul or anyone else ever disparage the tenets of faith, such as baptism, as though they were in the category of the works of the Law. They clearly were not. They are purely the expression of faith (Mark 16:15-16).