How do we know that one of the thieves was saved?

The certainty of the thief's salvation stems from Jesus' direct response during their final moments together. After acknowledging Jesus' lordship and innocence, the repentant thief asked to be remembered when Jesus comes into His kingdom (Luke 23:42). Jesus replied, 'Truly I say to you, today you will be with me in paradise' (Luke 23:43). This statement not only signifies immediate salvation for the thief but also reinforces the doctrine of justification by faith alone, as his belief was sufficient without works or sacramental participation, exemplifying God's grace and mercy even at the final hour.