Stephen Fry makes two points here. First, about moral relativism, and second, what is the point of the Church if she doesn’t condemn institutions.
Moral relativism is a great danger, and not at all for thinking people. This is because a thinking person (I would hope) would be appalled by female genital mutilation in Africa, or by the Ephebophilia of the ancient Greeks. A moral relativist would say these things are okay, because they are seen as okay within those cultures and contexts. An 8 year old girl who was married to a man 5 times her age is acceptable because in that culture it is acceptable. Moral relativism is therefore quite dangerous and not a benign belief. The Church, by offering constant teaching, allows us to judge other cultures and therefore protest such horrors as committed by Pashtun men on boys.
The other issue Mr. Fry makes is about the Church and slavery. As I’ve mentioned elsewhere on this blog, the Church rarely condemns institutions. This is in part because any institution might be made innocent if the hearts of people are properly ordered, and any institution might be made corrupt if the hearts of people are disordered.
For instance, if the hearts of people are disordered, communism can turn into totalitarianism. Or if the hearts of people are disordered, capitalism can turn into wage slavery. Both systems can become corrupted or can be pure. Slavery is a similar system. St. Paul wrote to Philemon to treat his slave Onesimus as a brother. If he were treated truly as a brother, then the system of slavery would be elevated into a good thing. It is from our hearts that systems are good or bad.
That said, Mr. Fry also simply doesn’t know his history. It was the conversion of the Emperor Constantine that initially did away with slavery because it was incompatible with Christian belief. The barbarians who set up camp after the empire collapsed didn’t frequently practice slavery, so it wasn’t an issue for many years. Eventually, when the issue popped up again, the Popes responded. Eugene IV wrote Sicut Dudum in 1435, condemning the slavery of Christians, before the new world was even discovered. Other encyclicals were written by other popes also condemning slavery as an institution. While admittedly the Bishops in America didn’t do much to encourage people to respect the papal writings, that doesn’t imply the Church didn’t speak out against slavery. It simply means the Bishops failed to live up to their responsibility. Given that Bishops are people too, this is understandable — especially with the severe anti-Catholicism that was happening at the time.
All together this is an issue of Liberation Theology. Mr. Fry condemns the Church for not attacking the structures of oppression and instead focusing on the hearts of people. While attacking structures is a good thing, the use of arms isn’t. In the deep south slavery would need to be overthrown by arms (the civil war), so the only option of the Church had open to her was to effect hearts. Otherwise she would have fallen into the heresy of Liberation Theology — that oppressors may legitimately be killed rather than converted. The Church has historically jumped at opportunities to condemn slavery without resort to arms, and has always (even from the Bible) strove to soften how slaves were treated in societies were slavery couldn’t be ended.