Chances are "vengeful" or "wrathful" were some of the first words to cross your mind. Why is that? Why has the God of the Old Testament become so associated with his wrath and, as usually follows, set in contrast to the "merciful" God of the New Testament?
Read Nehemiah 9 and "wrathful" is not the primary image of God you get.
This is one of the great chapters in the Old Testament: the people of Israel have just returned from captivity in Babylon, and they are recounting their past and confessing their sins to God. Notice the repeated references to their sinfulness and God's mercy:
"[Our fathers] refused to obey and were not mindful of the wonders that you performed among them ... But you are a God ready to forgive, gracious and merciful, slow to anger and abounding in steadfast love" (v. 17)
"They were disobedient and rebelled against you and cast your law behind their back and killed your prophets ... Therefore you gave them into the hand of their enemies ... And in the time of their suffering they cried out to you and you heard them from heaven, and according to your great mercies you gave them saviors who saved them from the hand of their enemies" (v. 27)
"But after they had rest they did evil again before you ... Yet when they turned and cried to you, you heard from heaven, and many times you delivered them according to your mercies" (v. 28)
"In your great mercies you did not make an end of them or forsake them, for you are a gracious and merciful God" (v. 31)
Don't overlook who is speaking in these verses. It is God's own people, Israel, who themselves have already suffered firsthand some of the worst of his judgments. And yet the dominant note of this chapter (and, really, the entire Old Testament) is how merciful and gracious and forgiving God is.
We are wrong to contrast the wrath of God in the Old Testament with his mercy in the New. He is the same yesterday, today and forever—terrible and righteous in his wrath, and swift and sweet in his mercy—whether before, on, or after the cross.