Objection 1: It seems that the God of the Old Testament is not the same God of the New Testament. Indeed in the Old Testament God seems more vengeful and in the New Testament he seems more merciful, as if they were two different divinities.
Objection 2: It seems there is no compatibility between the God of wrath and the God of love.
On the contrary, in John 1:1-6 is said: “In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things came to be through him, and without him nothing came to be. What came to be through him was life, and this life was the light of the human race; the light shines in the darkness, and the darkness has not overcome it.”
I answer that this kind of statement, that there are two gods, good and bad, is nothing new and indeed was at the heart of several heresies. But this was never the doctrine of the Catholic Church.
Let us consider, just an example, the Marcionites: “Heretical sect founded in 144 AD at Rome by Marcion and continuing in the West for 300 years, but in the East some centuries longer, especially outside the Byzantine Empire. They rejected the writings of the Old Testament and taught that Christ was not the Son of the God of the Jews, but the Son of the good God, who was different from the God of the Ancient Covenant. They anticipated the more consistent dualism of Manichaeism and were finally absorbed by it. As they arose in the very infancy of Christianity and adopted from the beginning a strong ecclesiastical organization, parallel to that of the Catholic Church, they were perhaps the most dangerous foe Christianity has ever known” (Arendzen, J. (1910). “Marcionites.” The Catholic Encyclopedia. New York: Robert Appleton Company). Also in the letters of St Paul (the oldest New Testament documents) the continuity between the Old Testament and the New Testament is clearly affirmed. For example he says in 1 Corinthians 10:1-12: “I do not want you to be unaware, brothers, that our ancestors were all under the cloud and all passed through the sea, and all of them were baptized into Moses in the cloud and in the sea. All ate the same spiritual food, and all drank the same spiritual drink, for they drank from a spiritual rock that followed them, and the rock was the Christ. Yet God was not pleased with most of them, for they were struck down in the desert. These things happened as examples for us, so that we might not desire evil things, as they did. And do not become idolaters, as some of them did, as it is written, ‘The people sat down to eat and drink, and rose up to revel.’ Let us not indulge in immorality as some of them did, and twenty-three thousand fell within a single day. Let us not test Christ as some of them did, and suffered death by serpents. Do not grumble as some of them did, and suffered death by the destroyer. These things happened to them as an example, and they have been written down as a warning to us, upon whom the end of the ages has come. Therefore, whoever thinks he is standing secure should take care not to fall.”
Reply to objection 1: Here there is probably a misconception that needs to be addressed. We are trying to interpret from what we know as if we have always understood the fullness of what God is. But we need to remember that God’s revelation is somehow progressive, so we understand Him more and more, even if we will never be able to understand Him fully. Indeed the famous saying (attributed to Saint Augustine) affirms that if we would be able to conceive Him fully in our mind, he would not be a God. So we should not make a mistake enclosing the immensity of God in the limited possibilities of our mind. Having said that, we cannot avoid also to mention that in the whole Bible God is referred as a compassionate God, a God full of mercy and love. So if we look more carefully it would not be so difficult to see that indeed the whole Bible is fully harmonized.
It is like for us, when in a given day we may be frustrated in the morning and happy in the afternoon: are we becoming two different persons or are these two moments of the same person?
Reply to objection 2: Also here, we should try to change the perspective we look at things. It is true that in the Old Testament God shows a certain side and in the New Testament another one. That is not difficult to understand when we consider that in the Old Testament there was the promise of the coming of a Messiah, a promise that will be fulfilled in the New Testament with the incarnation of Jesus. So the two Testaments are telling two different and important parts of the same story.