Look outward, at the promise
Assurance rests on Christ and the promise attached to him, never on the quality of your faith or your progress.
What it claims
The moment you make assurance depend on evidence in yourself, you have made it depend on you. And you are exactly the variable that fluctuates. Faith is not a quality you inspect; it is a hand that holds something. The promise is objective and outside you. God said it, attached it to Christ, and sealed it in your baptism. On bad days you cannot find your faith at all, and the promise does not move. This is why Luther told despairing people to stop examining themselves and to say instead that they are baptised.
Who holds it
Lutheran theology above all, and much Reformed preaching. Luther's own counsel was to look away from yourself entirely, and to remember your baptism.
The texts it stands on
- Romans 8:38-39 Nothing can separate us from the love of God in Christ, a list of external threats with nothing about the strength of our grip
- John 10:28-29 No one will snatch them out of my hand
- Titus 3:5 Not because of works done by us, but according to his mercy
- Hebrews 6:17-19 God guaranteed it with an oath, so that we might have strong encouragement
Strongest case
It is the only position that offers real comfort to someone in the dark. Every other view sends a struggling person back to inspect the very faculty that is currently failing. It also matches how the New Testament actually consoles people, which is by pointing at Christ rather than at their track record.
Hardest question
Jesus warns about people who say Lord, Lord and are turned away, and they clearly believed the promise applied to them. James says faith without works is dead. If assurance never involves looking at your life, what stops a person living any way at all while resting on a promise that was never theirs?
How it answers that
The warning in Matthew 7 is aimed at people trusting their own works, not the promise. Read it again: their defence is that they prophesied and cast out demons. They are pointing at their résumé. This view says that is exactly the wrong place to look, so the warning lands on the opposite error. And a promise genuinely trusted changes a person. Luther expected fruit; he simply refused to make fruit the ground of comfort, because the ground has to hold on the days when the fruit is not visible.