The first of the 95 thesis is incredibly poignant:
Our Lord and Master Jesus Christ, when He said Poenitentiam agite [Repent Ye], willed that the whole life of believers should be repentance.
Tim Keller writes about this:
On the surface this looks a little bleak! Luther seems to be saying Christians will never be making much progress. But of course that wasn't Luther's point at all. He was saying that repentance is the way we make progress in the Christian life. Indeed, pervasive, all-of-life repentance is the best sign that we are growing deeply and rapidly into the character of Jesus.He continues:
But in the gospel the purpose of repentance is to repeatedly tap into the joy of our union with Christ in order to weaken our need to do anything contrary to God’s heart.Be encouraged then, fellow believer. In calling you to daily repentance, the Lord Jesus is not simply giving you good advice. He is saying, "If you are a child of mine, you must continue to repent."
Many doctrines have deep roots in the Reformation but I think John Calvin's explanation of our salvation and the role we play in God's glorification is one of the most Biblically sound and fundamentally important in Church history. Here Calvin makes no attempt to obfuscate what could potentially be controversial because he has grasped the heart of the issue of our salvation; We have salvation through our faith in God's gracious sacrifice of Christ and his resurrection as propitiation for the purpose of His own glory. Below is a summary of what Calvin said.
Total Depravity - Sin has affected all parts of man. The heart, emotions, will, mind, and body are all affected by sin. We are completely sinful. We are not as sinful as we could be, but we are completely affected by sin and as a result separated from God.(Rom 3:9-18, Ephesians 2:1, 3,15, Rom. 6:6,14-20, 2 Tim. 2:26, Mark 7:21-23)
Unconditional Election - God does not base His election on anything He sees in the individual. He chooses the elect according to the kind intention of His will without any consideration of merit within the individual. Nor does God look into the future to see who would pick Him. Also, as some are elected into salvation, others are not. (Rom. 8-28-33, Ephesians 1:4-8, Rom 9:9-11, Rom 9:15,21)
Limited Atonement - This is concerned with the question of what did Christ do on the cross. It was there that he redeemed, justified, reconciled, and atoned for His chosen people. The debate on this issue is over the extent of the atonement. Did Christ die for all people, or just a few? If it is for all then the efficacy and efficiency of the cross is limited. If it is for a select people, then Jesus Christ’s death actually saves. This point is closely connected with the Reformation’s War Cry, “We are justified by faith alone!” (Matt 26:28, John 17:9, John 11:51-52, Rev 5:9, John 10:16, Hebrews 9:28)
Irresistible Grace - When God calls his elect into salvation, they cannot resist. God offers to all people the gospel message. This is called the external call. But to the elect, God extends an internal call and it cannot be resisted. This call is by the Holy Spirit who works in the hearts and minds of the elect to bring them to repentance and regeneration whereby they willingly and freely come to God. (Rom. 9:19-21, John 6:64-65, John 6:65, 1 Corinth. 1:23-24)
Perseverance of the Saints - You cannot lose your salvation. Because the Father has elected, the Son has redeemed, and the Holy Spirit has applied salvation, those thus saved are eternally secure. They are eternally secure in Christ. (John 6:47, Rom. 8:1, John 10:27-28, 1 Corinth. 10:13, Phil. 1:6)