Your assurance does not come from measuring your performance or your feelings. It rests on what Christ finished and what God promised, confirmed by His Spirit inside you. If you are trusting Him, you are His.
The Grace Answer
Most people who ask this have been handed a cruel test somewhere along the way: examine your life, and if your fruit is good enough, maybe you are truly saved. It sounds humble and serious. It is actually a trap, because on your worst day the evidence will always look thin, and your assurance will rise and fall with your mood, your energy, and your memory of last week. That is not faith. That is auditing yourself forever.
The Bible builds assurance on a completely different foundation. John wrote his letter for exactly this reason and said so out loud: “These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life.” He does not point you inward to your track record. He points you to a testimony outside yourself: “God has given us eternal life, and this life is in His Son. He who has the Son has life.” The question is not how well you have performed. The question is whether you have Him.
Where the confidence comes from
God does not leave you to figure this out alone. Paul says “The Spirit Himself bears witness with our spirit that we are children of God.” That quiet inner sense that you belong to Him, the fact that you even care about this question, the way your heart turns toward Him rather than away, that is the Spirit's own witness, not your imagination.
So assurance is not a feeling you manufacture and it is not a grade you earn. It is a promise you receive. If you are trusting Christ rather than your own effort, you are not on the outside hoping to be let in. You are already in, and the anxiety you feel is not evidence against you. Often it is simply a heart that has been trained to look at itself, learning for the first time to look at Him instead.