Karl Barth was the greatest theologian of the 20th century. He was also steadfastly unfaithful to his wife, Nelly.
Barth had his theological justifications for the affair. Something about the dialectical nature of truth. Proving it's possible to be smart and stupid at the same time.
For Barth, the Word of God is veiled in the fallible human words of Scripture. The Bible becomes the Word of God only through the subjective revelation of the Spirit.
If you’re smart enough and recognize no higher authority than yourself, you can justify anything. This is the great danger of moving away from radical obedience to the teachings of Jesus and the whole of Scripture. This is the folly of placing ourselves above the authority of God's Word. It's as old as Adam and as current as postmodernism.
Who am I to condemn Karl Barth? The words of Jesus do. He warned of the perils sexual sin. Pluck out your eye! Cut off your hand! Anything is better than facing God's judgment (Matt 5:27-30). If you're living a double life, put it right now.
Sin is serious stuff. It’s the reason he went to the Cross. That’s why the heart of discipleship is learning together to obey what Jesus taught (Matt 28:18-20). There's a price to pay and it's worth it.