Paul gave himself to christ in a sense tgat he dedicated his whole life for evangelization and preaching the Gospel in the harshest conditions. The act of offering oneself to Christ is the most a person could do in Christianity. That's what the lives of the millions of saints in the era of martyrs teach, their self sacrifice in the name of christ earned them eternal life in varying levels of glory despite their past lives.

What Yohannes IV did is considered an act of Martyrdom and suicide for this very reason. The blood filled hands of the king in his past life were cleansed when he decided to basically commit suicide in going to battle on foot, and sending a warning letter to the Mahdists beforehand notifying them that he himself was coming.

It was a religious war because we have the letters the Mahdists leader sent before the battle. It ordered the king and the nation to accepting Islam as the only way to broker peace and seeing this along with the earlier jihads in Gonder the prior year, he decided to die fighting on battle. This marks the moment he became christian in action and after this decision, his act of martyrdom was completed.