What does this justification do?
It grants us pardon for sin.
Catechism for the Global Methodist Church, question 59
6And he was faithful in the LORD, and he reckoned it to him as righteousness.
1Of David. A Maskil. Blessed is the one whose transgression is lifted, whose sin is covered. 2Blessed is the man to whom the LORD does not impute iniquity, and in whose spirit there is no deceit.
29The next day he saw Jesus coming to him and said, "Look, the Lamb of God, the one who takes away the sin of the world."
16For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek. 17For in it the righteousness of God is revealed from faith to faith, as it is written: The righteous shall live by faith.
6Just as David also says concerning the blessedness of the man to whom God counts righteousness apart from works: 7Blessed are those whose lawless acts were forgiven and whose sins were covered. 8Blessed is the man against whom the Lord will not count sin.
9Much more therefore, now that we have been justified by his blood, we will be saved through him from the wrath.
26Otherwise he would have had to suffer repeatedly since the foundation of the world; but as it is, he has appeared once for all at the end of the ages to do away with sin by the sacrifice of himself.
18You know that you were redeemed, not with perishable things such as silver or gold, from the futile way of life handed down from your forefathers, 19but with the precious blood of Christ, as of a lamb without blemish or spot.
24Who himself carried up our sins in his body upon the wood, that, having died to sins, we might live to righteousness; by whose wound you were healed.
2And he himself is the propitiation for our sins, and not for ours only but also for the sins of the whole world.
10In this is love, not that we have loved God, but that he loved us and sent his Son as the propitiation for our sins.
The catechism has just asked how sinners are justified; this question asks what justification accomplishes. The answer comes from ¶102 of the Book of Doctrines and Discipline, where the fifth stage of the Wesleyan way of salvation reads: "God's justifying grace is received by faith to reconcile us to God through the atoning sacrifice of Jesus Christ, what God does for us. It is pardon for past sins."
Pardon is a legal word, and the scripture references keep it that way. David, looking back on his own forgiveness, sings, "Blessed is the one whose transgression is lifted, whose sin is covered. Blessed is the man to whom the LORD does not impute iniquity" (Psalm 32:1-2). Paul takes up that psalm to describe "the blessedness of the man to whom God counts righteousness apart from works" (Romans 4:6), and behind both stands Abraham, who "was faithful in the LORD, and he reckoned it to him as righteousness" (Genesis 15:6).
The pardon is grounded in the cross and nowhere else. John points at Jesus and calls him "the Lamb of God, the one who takes away the sin of the world" (John 1:29). Peter reminds his readers: (1 Peter 1:18-19) Christ is the one "who himself carried up our sins in his body upon the wood" (1 Peter 2:24). John names him "the propitiation for our sins" and widens the scope to "the sins of the whole world" (1 John 2:2), the conviction ¶102.1 states when it says Christ died for all people. The same letter locates the whole matter in God's initiative: (1 John 4:10)
A Methodist who has received this pardon can say so without hedging. The gospel "is the power of God for salvation to everyone who believes" (Romans 1:16), and those "justified by his blood" will be saved "from the wrath" (Romans 5:9). The past is dealt with by God's own verdict, which is why the prayer of confession in Methodist worship ends in a declaration of forgiveness. What remains is the question the catechism asks next: how a person can be sure of it.