Every religion with some kind of concept of sin has to address how to be seen as just, or righteous, in the eyes of God. Maybe a simpler way to explain it is with this question: How can we make up for our sins?
For all their differences, the vast majority of religions have a shared conclusion about how we become righteous. And the key to righteousness is typically some kind of combination of faith and works, with the latter usually being more important than the former.
And then there’s what Christians believe. Christians don’t believe our works are a means of earning God’s favor because there’s no way for us to offset the offense of our sin against a perfect and holy God. Instead, we understand that justification comes to us one way, and one way alone, which is through faith in Jesus Christ, the only truly righteous Person, the only Person who perfectly obeyed God in every way.
That’s what Christians mean by the term justification by faith: A person is declared objectively righteous before God, not based on personal merit but by faith in the righteousness of Christ (Rom. 8:33-34; Eph. 2:8-9).
Righteousness, or justification, is something none of us can earn but is freely given to all who trust in Christ. And through our justification, we are pardoned and declared to be in right standing before God, no longer estranged but adopted into His family.