The dictionary defines the word justify as "To declare guiltless or blameless". This word as it is used in a Biblical way, "To regard and treat as righteous on the ground of Christ's mediatorial work".
The Apostle Paul said in Romans Chapter 5, verse 1, "Therefore being justified by faith, we have peace with God through our Lord Jesus Christ". He continued his message to the people in Rome by stating in Romans Chapter 6, verses 17 and 18, "But God be thanked, that you were the servants of sin, but you have obeyed from the heart that form of doctrine which was delivered you. Being then made free from sin, you became the servants of righteousness".
Sin is the only thing that will separate man from God. Romans Chapter 3, verses 23 through 26, "For all have sinned, and come short of the glory of God; Being justified freely by his grace through the redemption that is in Christ Jesus: whom God hath set forth to be a propitation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God; To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus".
When a person comes to realize he is a sinner and is separated from God as the result of sin, he is in the position to look for the way to be justified with God or to have sins forgiven in order to be regarded as righteous on the ground of Christ's mediatorial work.
The writer of the Roman letter, the inspired apostle, stated clearly that being justified by faith and making peace with God will be realized when our faith is great enough to cause us to obey from the heart the form of doctrine delivered. (Romans Chapter 6, verses 17-18)
The doctrine delivered is the death, burial and resurrection of Christ. In I Corinthians Chapter 15, verses 1 through 5, "Moreover, brethren, I declare unto you the gospel which I preached unto you, which also you have received, and wherein you stand; By which also you are saved, if you keep in memory what', I preached unto you, unless you have believed in vain. For I delivered unto you first ol' 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".
The doctrine was the death, burial and resurrection of Jesus. One cannot obey the facts of the gospel, but one can obey a form of his death, burial and resurrection. When one has come to believe that Jesus is God's son and the saviour of man from his sins, he being moved by faith to repent of his sins, that is, he determines to discontinue to live in sin, he being willing to confess that faith in Christ as beina God's son is a proper subject to be buried in baptism for the remission of sins, then raised a new creature.
In Acts Chapter 2, the Apostle Peter talking to the ones in sin told them they had crucified the Son of God. They were concerned about their condition, thereby showing faith. They asked in Acts Chapter 2, verse 37, "Men and brethren, what shall we do?". In verse 38, "Then Peter said unto them, 'Repent and be baptized every one of you in the name of Jesus Christ for the remission of sins, and you shall receive the gift of the Holy Ghost"'. In Acts Chapter 2, verse 41, it states, "Then they that gladly received his word were baptized". And in verse 47, we see the blessing received. It says they were "praising God, and having favour with all the people. And the Lord added to the church daily such as should be saved".
Their being saved is equivalent to being justified with God as to be justified means to be declared guiltless or blameless or to be regarded as righteous on the ground of Christ's mediatorial work.