People struggle with the idea that the God of the Old Testament, which they see as this angry wrathful God, is the same God as the God of the New Testament, who is supposed to be a God of love and mercy. They cannot reconcile how the two are compatible. What people do not realize, is that the God of the Old Testament is as much a God of mercy as the God of the New Testament. One has only to read the Psalms to realize this. The God of the New Testament is a God of vengeance and terrible wrath, which one can see, if one reads the book of Revelation. So we see that God is both merciful and wrathful in both Testaments, but that does not answer the problem of how a God of justice, love, and mercy can be one in the same as the wrathful God who is also portrayed. In actuality the answer is simple. The reason God is a God of wrath is because He is a God of mercy. If He were only a just God, He would be neither merciful nor wrathful. Let me explain this more clearly.
We know God is supposed to be a God of justice. A loving and fair God could only be a God of justice. To be a God of justice, there must be appropriate punishments for appropriate sinful actions. When a person commits a sin, their fate is already a fact, as the punishment for that sin is already decided. Committing that sin requires that punishment. A God that is strictly just will simply apply the punishment deserved for the sin committed. There will be no mercy, nor wrath, simply justice. Here's the crime, here's the punishment. Period. A God of justice knows that justice has to be served by having the punishment dealt out for the crime. If a God of mercy were to absolve the person of punishment for the sin, then He would no longer be a God of justice, for someone has to pay the penalty for the sin for justice to be served. Therefore a just and merciful God, in order to offer mercy to someone, must therefore provide a way for justice to be served.
God decided to provide a way for that to happen. He offered His only Son to take the punishment for all sins, that was necessary for justice to be served, thereby freeing Him to offer mercy. So when we stand before the judge (God) (some people make the decision to accept God's offer now, and some stand in judgment after death to receive their punishment) and our crimes are listed and the punishment decided upon, then God, out of the goodness and love of His heart, offers us mercy. We do not deserve this mercy, we have not earned this mercy, we deserve the punishment that justice demands, however He has provided that justice will be served in another way, (Christ's atonement) and offers us the mercy. For those who accept it, we are freed from the punishment of those sins. There are those though, who instead of accepting this free gift, which we do not deserve, throw it back in God's face and spit upon Him. They do not want His gift, they do not accept their punishment as being deserved, they sometimes do not acknowledge that He even has the right to judge them. And they mock Him, and challenge Him, and blaspheme and show contempt for the Son, who has already taken their punishment upon Himself, saying to God that He should do His worst; they will take their own licks, thank you very much.
Now put yourself in God's place. If you offered your only child and punished them with every punishment that anyone ever deserved, and then when you offered this incredible gift of mercy to those who deserved this punishment, they threw it back at you, blasphemed you and your child, and stomped on that precious gift, what would your response be? God's response is, okay, you will get your punishment that you so richly deserve, and beyond that, I will punish you, not only the minimal amount of punishment that I must deal out for your sin, I will punish you to the full extent that the law will allow, because you have treated My mercy and My Son's sacrifice with such contempt. This is contempt of court. This then is God's wrath. This is God's justice. This is God's mercy. This is God's love and vengeance (this justice must too be served) for His Son, who has now been treated with such bitter contempt. It is this added sin that allows God to pour out His wrath and still be just in so doing.
Many people believe that they can find their own way to God, but if God is a God of love, justice, mercy and wrath, that is not possible. Justice must be served. He is the only one who could provide the means for mercy to be bestowed, while justice would still be served. Nobody else except Christ has offered to serve that justice. No other religion has offered us a substitution for our punishment, so that justice and mercy could both exist side by side. That is why Jesus is the way, the truth, the life, and nobody can come to the Father but through Him. He is the only one who has served justice on our behalf, so that God can offer us mercy. To reject the fact that Christ is the only way, the one provided by the judge, is to throw that mercy back in God's face and spit on Him. Anyone who does this should expect that one day they will be the recipient of the full force of God's wrath.
This is why God is Just (He provided that justice is served for all sins), Loving (He loves us so much that He gave His only Son to be our substitute when He did not deserve our punishment), Merciful (He offers that gift of mercy freely to anyone who wants it regardless of how bad their sins may be), and Wrathful (justice is served not only on a person's sin, but on their rejection of that incredible mercy that is not deserved) at the same time. If you have not accepted this mercy, do you not think it is time you did?