How do we know that salvation is a completed work?

The finished work of Christ on the cross is central to Reformed theology, echoing the message of John 19:30, where Jesus declared, 'It is finished.' This signifies that the debt for sin was fully paid, making salvation complete for all who believe. In Acts 20, Paul’s preaching emphasizes the finished state of salvation in Christ, indicating that we don't maintain our salvation through our efforts but rest in His completed work. This understanding alleviates the burden of trying to earn God's favor, showcasing that salvation is solely by grace, through faith in Christ, assuring believers of their eternal security as expressed in Philippians 1:6 — that He who began a good work will carry it on to completion.