Martin Luther said that Justification was the doctrine "by which the church stands or falls".
What is justification, (or as it is sometimes translated as "being declared righteous")? Who is justified? Who is the one doing the justifying? Why is there a need for justification? What happens at justificaiton?
Justification, as far as I can tell from the Bible, is a legal act God, which in free grace to sinners, he fully pardons all their sins (past, present, and future). Like a judge, God declares them "Not Guilty". The reason He accepts and considers them righteous in his sight is not because of any thing done in them, or done by them, but it is only because of Christ's perfect obedience and full satisfaction to God's justice he made on their behalf. Christ lived the life we couldn't live and died the death we should have died. God counts the righteousness of Christ as theirs, and credits it to their accounts. This is sometimes called "the great exchange". My sins go to Christ, his righteousness goes to me.
Justification is received by faith alone. What happens is a sinner is convinced of their sin and misery, and of their inability to get out out of this lost condition. They hear the good news, then the sinner not only assents to the truth of the promise of the gospel, but receives and rests upon Christ and his righteousness alone for salvation. This faith is a gift from God. It is an instrument by which we receives Christ and his righteousness.
The WT Society, and the Catholic church both agree that this position is wrong, and that Justification is not by faith alone, but Faith and works combined. However, good works, are the fruits and effects of this type of faith. Paul stresses this three times in one verse:
"Yet we know that a person is not justified by works of the law but through faith in Jesus Christ, so we also have believed in Christ Jesus, in order to be justified by faith in Christ and not by works of the law, because by works of the law no one will be justified." - Galatians 2:16
Salvation from beginning to end is all of God's grace. Humans try to insert themselves into the process so many times and in so many different ways. But that is completely not what Paul or the rest of scripture says.
"But when the goodness and loving kindness of God our Savior appeared, he saved us, not because of works done by us in righteousness, but according to his own mercy, by the washing of regeneration and renewal of the Holy Spirit, whom he poured out on us richly through Jesus Christ our Savior, so that being justified by his grace we might become heirs according to the hope of eternal life." - Titus 3:4-7