Martin Luther argues that Christians can have assurance of their salvation based on three grounds: 1) The promises of Scripture that those who trust in Christ alone are saved. 2) The sanctifying work of the Spirit seen in the lives of believers. 3) The witness of the Spirit causing believers to cry "Abba, Father." Luther says this cry, though small, expresses the believer's knowledge that they are God's child through Christ. He urges believers to train their conscience to believe that God approves of them, despite doubts, based on Christ's righteousness.