In the last bit of John’s letter, he goes back to the main concern he has had from the opening of the letter. Those in the light are being challenged and troubled by the voices of those still in darkness.
To address the claims of those who would deny Jesus, John references the people to three events.
Water = Jesus’ baptism
Blood = Jesus’ death and resurrection
Spirit = the coming of the Holy Spirit in Acts 2
John uses these three public instances to show that Jesus’s life was real. People saw John testify about him, people knew he was crucified and that so many saw him after his death, and finally the work of the Holy Spirit that started at Pentecost and has continued in the life of every believer since.
John says our greatest advocacy to those around us for the reason we have to hope in Christ is the life we live in this current state. When we turn the other cheek Jesus is seen. When we give our coat and our shirt to those in need, we show others Jesus. When we love our enemies, the way christ loved us, his life changing power is seen on display in us.