There's only one God. And He wrote both.
But the point to be made is that if we take seriously the eternal perspective that is laid out in the New Testament, then it simply will not do to write off the Old Testament witness as intrinsically harsher and therefore not something we need worry our heads about today.
I think it is closer to the truth to say that in the coming of the Lord Jesus and the new covenant he sealed with his own blood, both the justice of God and the mercy of God appear in sharper relief than ever before, leaving us with correspondingly less excuse, and with great grounds for praise and worship.
—D. A. Carson, How Long, O Lord? Reflections on Suffering and Evil, 1st ed. (Grand Rapids: Baker, 1990), 105.
HT: Justin Taylor