“Written in blood” usually conveys ones willingness to stand behind their words with their life.
When Paul ended his letter to the Galatians, he was, in a figurative sense, writing his own story in blood. He wrote a message of love and grace that would arouse the anger of the religious leaders of his time. He knew he would be hated for honoring the death of Jesus Christ above the ritual and moral law of circumcision that represented the whole Mosaic way of life. His suffering for Christ would literally include shedding his own blood.
Paul wasn’t willing to play it safe. This is because he knew that the crucifixion of Jesus Christ was the center stage of history. He decided to put his own life on the line, Paul openly proclaimed the inexpressible heart of God, who gave His Son to express the ultimate words of love written in blood on the Cross.
To show His love, Jesus died for each of us and to show our love, we must live for Him!