Actually, our justification was accomplished by His resurrection, not His death (Romans 4:25) and this point is important. Nevertheless, I am going to include our justification under the reasons Christ died, since His death and resurrection cannot rightly be discussed apart from each other anyway, and since, after all, you can’t be resurrected unless you have first died.
One effect of Jesus’ death was that He thereby escaped from the jurisdiction of the Law. For the Law only “has dominion over a man as long as he lives.” (Romans 7:1) But Christ died. And then He rose again, but what does the Law know of that?
And the extra wonderful thing for us is that in Holy Baptism, Christ permits us to follow Him through that freeing death and into a whole new realm, His resurrection Life, the Kingdom of God, the realm of the Holy Spirit, heaven.
Or do you not know that as many of us as were baptized into Christ Jesus were baptized into His death? Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life.
For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection, knowing this, that our old man was crucified with Him, that the body of sin might be done away with, that we should no longer be slaves of sin. For he who has died has been freed from sin. Now if we died with Christ, we believe that we shall also live with Him, knowing that Christ, having been raised from the dead, dies no more. Death no longer has dominion over Him. For the death that He died, He died to sin once for all; but the life that He lives, He lives to God. Likewise you also, reckon yourselves to be dead indeed to sin, but alive to God in Christ Jesus our Lord. (Romans 6:3-11)
Death has no more dominion over Christ, and no more dominion over us who dwell safely in His bosom. And if death has no more dominion over us, what can the Law do to us? And if we are members, even, of the all-righteous Christ, "of His flesh and of His bones" (Ephesians 5:30), what charge can the Law even bring?
Or do you not know, brethren (for I speak to those who know the law), that the law has dominion over a man as long as he lives? For the woman who has a husband is bound by the law to her husband as long as he lives. But if the husband dies, she is released from the law of her husband. So then if, while her husband lives, she marries another man, she will be called an adulteress; but if her husband dies, she is free from that law, so that she is no adulteress, though she has married another man. Therefore, my brethren, you also have become dead to the law through the body of Christ, that you may be married to another--to Him who was raised from the dead, that we should bear fruit to God. For when we were in the flesh, the sinful passions which were aroused by the law were at work in our members to bear fruit to death. But now we have been delivered from the law, having died to what we were held by, so that we should serve in the newness of the Spirit and not in the oldness of the letter. (Romans 7:1-6)
We are sinners; but Christ shares with us His own, righteous Life and the Law cannot even charge us. We deserve to die; but Christ has already given us His own, eternal life and death cannot touch us. This is how Christ justifies us: by giving us Life, by being our Life.
By being the Life of our Life and the righteous Heart of our hearts, He justifies us, at the same time both fulfilling the Law in us and *making it all moot.*
Christ “wiped out the indictment that was against us, which was contrary to us. And He has taken it out of the way, having nailed it to the cross.” (Colossians 2:14) He simply took it out of the way. He cancelled it.
There is therefore now no condemnation to those who are in Christ Jesus, who do not walk according to the flesh, but according to the Spirit. For the law of the *Spirit of life* in Christ Jesus has made me free from the law of sin and death. For what the law could not do in that it was weak through the flesh, God did by sending His own Son in the likeness of sinful flesh, on account of sin: He condemned sin in the flesh, that the righteous requirement of the law might be fulfilled in us who do not walk according to the flesh but according to the Spirit. (Romans 8:1-4)