What is the New Testament basis for the idea the God is a God of judgment and vengeance? The New Testament is frequently seen as presenting a God of love, compassion and mercy, while the Old Testament is often viewed as presenting a God of judgment and vengeance.  However, the Bible collectively presents God as unchanging.

What, then, is the New Testament basis that supports the idea that God is a God of judgment and vengeance?
Let's start with Matthew 25:31-46 where Jesus talks about Judgement Day whereon He will separate the sheep from the goats, casting the goats into Hell.

In Romans 12:18-20, Paul cites the extant scriptures for why Christians should not take their own vengeance:


  If possible, so far as it depends on you, be at peace with all men. Never take your own revenge, beloved, but leave room for the wrath of God, for it is written, "VENGEANCE IS MINE, I WILL REPAY," says the Lord. "BUT IF YOUR ENEMY IS HUNGRY, FEED HIM, AND IF HE IS THIRSTY, GIVE HIM A DRINK; FOR IN SO DOING YOU WILL HEAP BURNING COALS ON HIS HEAD."


In Revelation 2, Jesus dictates a letter to Pergamum wherein He says, "Therefore repent; or else I am coming to you quickly, and I will make war against them with the sword of My mouth."

Substantial portions of the rest of Revelation discuss God's impending judgement on the world.

In Acts 5, we see God issuing immediate punishment to Annanias and his wife Saphira for conspiring to lie to Him. In Acts 12, God strikes Herod for not giving Him praise, "And immediately an angel of the Lord struck him because he did not give God the glory, and he was eaten by worms and died."

Jesus Himself cleanses the temple of money changers by making a whip and driving them out [John 2:15].

There is no "inconsistency" between the two "halves" of the Bible - it is truly one book, the record of God's work and plan of salvation throughout history.