St Anselm said that Jesus’ obedience ‘paid a ransom’ for the sins of all humans. He was using a metaphor here from the slave trade. Just as slaves could be freed if enough money was paid, so humans are freed from the effects of their sins by the obedience and death of Jesus. 

God’s justice required that Jesus suffered. Jesus offered up his life in love of God and in love for all humans. This love far outweighs the rejection of God caused by human sin. 

Some Christians are unhappy with the parallel of a random being paid in St Anselm’s metaphor. They question to whom the ransom is paid, which would imply there is someone greater than God. However, this is not what St Anselm was trying to say. It was God’s honour that had been denied, and this has to be respected by total obedience.