Assurance vs. Argument

List one verse that assures you of your salvation. And I wrote: 1 Corinthians 1:9.
"God is faithful, by whom you were called into the fellowship of His Son, Jesus Christ our Lord."
It surprised them, I think, that this was my go-to verse. But it is the character and the faithfulness of God that is the key issue to assurance of our salvation...shhhh...that both Calvinists and Arminians seem to often miss.
On one side, the Calvinists lean hard on their "chosen" status -- not the character of the Chooser. But the doctrine of "election" is a human interpretation of God's divine revelation of who is saved and how we are saved. As a human interpretation, it is not a safe landing place in the face of fear and doubt. 
On the other hand, Arminians lean hard on their "choosing" of Christ -- not the character of the Chosen One. Man's work nor thoughts can never be the pillar of our confidence -- because we fail... or we wouldn't need a Savior to begin with.
But either way you swing, it is wide of the mark. It is the unyielding truth of Who God is where our assurance resides. He is trustworthy, He is faithful, He is able to save to the uttermost those who come to God (Hebrews 7:25). 
Rest in Him.