If God is forgiving (Neh 9:17), why doesn't he just forgive everyone? Why did he need a sacrifice to compensate for our sins? Aren't atonement and forgiveness in some way opposed?

When the Bible says God is forgiving, does it mean that he is forgiving via the atoning sacrifice of Jesus? Or does it mean that he forgives some sins, but others need atonement? Or does it mean that he forgives any number of individual sins, but generally sinfulness needs atonement to be nullified?