How do we know Christ's sacrificial work is sufficient for salvation?

The sufficiency of Christ's sacrificial work is a cornerstone of Reformed theology and is illustrated in Joseph's story, where Joseph acts as a deliverer despite the wrongs against him. Similar to Joseph's role in preserving lives, Christ saves His people fully and entirely through His substitutionary sacrifice. In His cry, 'It is finished' (John 19:30), we find assurance that there is nothing left for us to do but believe. The idea that we could add our efforts to Christ's work undermines the greatness of His atonement and ignores the divine plan of grace that secures salvation for the elect.