Simply believe what the lost people in Corinth believed when God declared them "not guilty"...
For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures, and that he was buried, and that he rose again the third day according to the scriptures. (1 Corinthians 15:1-4)
Believing those facts result in being justified by God. But what exactly does justified mean?
To be justified means more than to be shown mercy or to be forgiven, though that is certainly included. To be justified by God is to be acquitted or declared "not guilty" of all transgressions against Him, thus counted by Him as righteous.
Paul explains on how the sinner's justification is accomplished in his letter to the Roman believers by using the example of Abraham, whose simple faith in God's promise was credited to him as righteousness:
...that "it [righteousness] was credited to him" [to Abraham] was not written only for Abraham’s sake but also for our sake [that includes you and me], to whom it will be credited: those who believe in the One who raised Jesus our Lord from the dead. He [Christ] was given over because of our transgressions and was raised for the sake of our justification.
So according to the apostle Paul, those who are justified by God are justified on the basis of simple faith in what He is asking all of us to believe: that Christ died for our sins, was buried and rose again.
Now at this point of the discussion three questions frequently come up:
1. Okay, maybe I'm not perfect but I'm not THAT bad, either. A lot of people in the world are WAY worse than I am so why do I need to be justified?
Because of Adam's sin. Adam was the physical father of us all, and God looks at all of lost humanity as part of Adam: “By one man [Adam] sin entered into the world, and death by sin, so that death passed upon ALL men because ALL sinned" (Rom. 5:12). Adam's sin counts against all of us, for we all were yet in him when he sinned.
So it's not so much a matter of what we do, but what we ARE. Sinning doesn't make us sinners. We sin because we're sinners. By nature, we already stand filthy, unjustified and condemned before a holy God. In fact, because of sin, God bluntly refers to us as His enemies (Romans 5:10).
That we all sin, violate our consciences (the Law of God which He placed in our hearts) and ultimately die is all the proof we could ask of our own sin, should we doubt it. Many professing Christians do not grasp, agree with or like this fact, but God's Word says it is a fact.
2. How can God declare a lost sinner forgiven, justified and even righteous simply by his or her faith? Didn't He demand all sin be paid for by blood?
Yes He did, and He does it by the blood of the Cross. Upon a person's faith in Christ - meaning belief in His death, burial and resurrection from the dead for our sin - God no longer identifies him or her with Adam, the fallen, sinning father of our flesh. Instead, the believer is declared legally DEAD and his or her new life is the life of Christ, the second Adam (1 Cor 15:45):
I am crucified with Christ: nevertheless I live. Yet not I, but Christ liveth in me, and the life which I now live in the flesh, I live by the faithfulness of the Son of God, who loved me and gave Himself for me. (Galatians 2:20)
This is what the Bible means when it speaks of how believers are baptized [placed; identified] into Christ. This baptism has nothing to do with physical H2O but is the "one baptism" by the Spirit into the Body of Christ (Ephesians 4:5) at the moment of faith. At that moment, the believer is fully and forever identified with Christ. As a result,
...we have redemption through His blood, the forgiveness of sins according to the riches of His grace (Eph. 1:7).
3. But don't we have to do something in addition to faith, to show God we're worthy of, or have earned, or to keep His justification?
The apostle Paul - the man through whom Christ chose to reveal these truths - answers that question repeatedly with an emphatic no. Salvation today is by grace through faith alone in Christ, without works to obtain, maintain or prove one's salvation (Ephesians 2:8-9). Why? Because we do not deserve it and there's nothing we can do to earn it. God's "not guilty" comes to us through faith in Christ alone, expressly without any work or effort on our part. Christ did ALL that God ever required to bring you and I to Himself, so ALL the glory for our salvation must and should go to Him alone.
Since the work of our salvation has been completed, there's nothing left for us to do except that which God says to do:
That's why, when the sin-convicted Philippian jailor begged of Paul "What must I do to be saved?" the apostle's answer was not a list of sacraments, a litany of rituals, a checklist of good deeds that must be done, a prayer to pray, a promise to make to God, nor warnings of sins to stop committing first. No...the Good News could not have been any simpler:
"Believe on the Lord Jesus Christ and you shall be saved!" (Acts 16:31)
If you're not 100% sure God has declared you justified in Christ, what's stopping you from, right now, being forever acquitted of all your sin and having your conscience forever cleansed by God Himself?
Receive not the grace of God in vain. . .Behold, now is the accepted time; behold, now is the day of salvation. (II Cor. 6: 1-2)
He cannot lie and has given you His Word: simply believe on the Lord Jesus Christ and you will be saved!