In this simple question, there are two very important theological words that should be understood— justified and faith.
1/ The word, justified, essentially means a person’s sins have been accounted for (remitted) in the eyes of God and that the person is no longer under the condemnation of them. Implied in the expression is the notion that justification occurs by a divinely prescribed means. It is clear from the book of Acts what that means is in the Christian era, namely, baptism (Acts 2:38; 22:16). This is the experiential way to be justified. Theologically, it can simply be expressed as, by faith, since it is faith in Christ that leads a person to be baptized (Mark 16:15-16).
2/ The word, faith, likewise, is highly theological. It has various meanings depending on its theological context. For example, sometimes we refer to, the faith, as Christianity in general. Other times, it means faith by itself (James 2:14; Mark 4:40). Mostly, it means faith that actuates, as we see in the honor role of faith in Hebrews 11. In the many examples provided there, faith is clearly seen to be what prompted righteous action.
So, back to the question: Aren’t we justified by faith?
The answer is: yes and no, depending on a person’s theological use of the word, faith. Experientially, a person is not saved by faith (alone) but by the obedience of faith to the terms of salvation (Romans 1:5; 16:25). Upon hearing the Gospel, a believing person is prompted by faith to obey its saving tenets (Mark 16:15-16), which result in justification (Acts 2:38; 22:16). Therefore, theologically, a person can be said to be saved by faith since it initiated obedience to the tenets of salvation. This is how the Apostle Paul used the expression in his epistles. All the epistles in the New Testament were written by and to persons who had previously received the Acts 2:38 salvation experience.
From this understanding, we can conclude that there is an experiential and a theological way to view justification. The justification we see take place experientially in the book of Acts occurred through repentance, water baptism in Jesus’ name, and the infilling of the Holy Ghost. But all of this was prompted by faith in Christ. Hence, the basis for obedience to Gospel terms is faith. Therefore, references to that salvation in the epistles were theologically abbreviated to justification by faith. It was perfectly understandable to those to whom Paul and others were writing because of their common salvation experience.
Oftentimes, when Paul referred to justification by faith, he was contrasting salvation in Christ with attempting salvation by the works of the Old Testament Law or some aspect of the Old Covenant (e.g., Galatians 3:21-29). He, therefore, stressed that Christians are justified by faith. Theologically speaking, faith refers to our confidence in the salvation provided by Christ as opposed to reliance on any other means or any additional means. Paul never meant that we are justified simply because we believe in the Person of Christ. This is clear by what we observe of actual Christian salvation in the book of Acts, including Paul’s own conversion (Acts 9; 22:16) and ministry practice (Acts 19:1-6).
Faith in Christ means three things in the Christian era:
1/ Faith in Jesus as the long-awaited Jewish Messiah.
2/ Faith in His atoning sacrifice for our sins.
3/ Faith in what Christ prescribed for salvation.
The principle of salvation in the New Testament is faith in Christ (John 3:16; Acts 16:31; Ephesians 2:8-9). That is not the plan of salvation but it is the principle of salvation. Justification by faith in Christ simply means that we are saved by what Christ prescribed for salvation rather than by any other means. 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. Therefore, the tenets of salvation that are contained in it are mandated by Christ for use in evangelism. On the Day of Pentecost, Peter spoke as a commissioned minister and preached the plan of salvation that every New Testament minister is commissioned to preach.
Acts 2:38 is how we are justified by faith. And all the examples of salvation in Acts bear it out.