Evidence of Salvation: Light
Salvation is all of God. He planned, He pursued, He provided and He will bring it to perfection. Either you are saved, at this moment, or you are not. There are no partially saved people. Salvation is base on the object truth of what God has done in Christ. So when it comes to your assurance of salvation, the real question you are asking is what is the evidence of the objective truth?
That is why we find ourselves in the letter John wrote to a group of churches who were facing some doubts because of dissenters who where distorting the gospel. John, the beloved disciple, who also wrote the Gospel of John now is writing to give confidence not just in the message of the salvation but also in the measure of assurance, the evidence, of that salvation.