God must be true to his character. God is light, there is no darkness found in God. In Jewish practice, Yom Kippur is the day of atonement. The high priest would go into the holy of holies to smear blood on the altar for the sins of the whole people. What it accomplished is John s point it erased, neutralized, cancelled the darkness of the sin.
Expiation means to neutralize or cancel sin.
It happens through the blood of the cross. It is because of the blood of Jesus on that cross, that we are made whole. He comes alongside as sacrifice because we need Jesus as atoning sacrifice. Now the problem is there are about 30 messages to preach from this section and there is theology here that runs amazingly deep that whole courses are taught on it. But John isn t giving a theology lesson he s talking to people to help keep them from sin.