We could have disturbing doubts about our salvation. Such doubts are most often because we focus upon ourselves and not on Christ. Trusting in ourselves leaves room for all manner of doubt. Salvation without doubt is focusing upon Christ alone. It's knowing who saved you.
"Believe on the Lord Jesus Christ, and you will be saved" (Acts 16:31). Salvation without doubt is focused upon what Christ has done for us at the cross and through his resurrection (1 Corinthians 15:1-4). We are saved by what Christ is doing in us and through us (Colossians 1:27; Hebrews 13:20-21). In Christ alone we are saved. Salvation is sure, believing and trusting in Christ (Ephesians 1:13-14).
Focused on Christ alone is the way to salvation without doubt. Then, we can say, "I know whom I have believed and am persuaded that he is able to keep what I have committed to him until that Day" (2 Timothy 1:12). Salvation is sure by faith in Christ alone. He alone has the authority and power to save us and keep us.
Salvation without doubt is focused upon Christ alone, and his work for us and in us. The grace of Christ initiates our salvation. What he begins, Christ is more than able to finish. "Being confident of this very thing, that he who has begun a good work in you will complete it unto the day of Jesus Christ" (Philippians 1:6).