One of the commoner themes of the modern atheism is the notion that the Christian God is a monster; buttressing their pre-existing conviction that it is so with appropriate verses from the Old Testament, and adding a few curlicues from their literalist view of hell they construct an edifice of their own devising which ignores what Christians, whose book the Bible is, after all, have had to say on the subject. Satisfied with their own cleverness, and uninformed by any other readings of Scripture, such atheists are clearly reacting to something within themselves rather than to the reality of the God known to Christians.
It is almost possible to understand why, in the Catechetical Lectures of St. Cyril of Jerusalem, catechumens were not allowed to read the Sacred Texts. This was not, as some ill-informed souls would have it, because priests were trying to keep knowledge from the people, but rather the result of the belief that, read without proper catechesis, people would misunderstand what they were reading; the modern Internet atheist often seems bent on showing the wisdom of such a line.
Oddly enough, we all know what is in the OT. But what we also know is that it must be read in the light of the New Testament. If we take it purely in its own terms, we read it as faithful Jews do. But we are not devotees of Judaism, we are Christians. So we read the passages so beloved of our mocking friends in the light of Christ.
Oddly enough, we are able to distinguish between what the Jews of old believed about God with the self-revelation offered in the NT. So, whilst it was natural for those Jews to see the Father as a vengeful tribal God, that reflected their own limitations, not those of God. Only in the Christian revelation do we see His True Nature.
Atheists seem a little shocked to be told that the Bible needs to be read as a whole and can’t just be chopped up to be used as polemic. The revelation offered us by Christ shows God as the Father in the story of the Prodigal Son; no slayer of His foes on offer here.
Christ did not save us by coming in Power and smiting His enemies, but by being the suffering servant who took on Himself our sins. That is the Christian God, and that is why we believe in Him, love Him, and worship Him. If our atheist challengers would take a few moments away from lecturing us to educate themselves, it would be better for them.