There were three famous "solas" (Latin word for "only") that Martin Luther coined, which ultimately sparked the Reformation. Here they are below, as well as a description of each:
1. Sola Gratia ("only by grace alone"): Luther read scripture verses such as Ephesians 2:8 ("For it is by grace you have been saved..." ESV) and came to believe that it is only through God's grace, not any works a person can do, that salvation is accomplished. One must rely on God's free and infinite grace for his salvation and must not try to add to it through good deeds, giving money, etc.
2. Sola Fide ("only through faith alone"): Luther furthermore believed whole-heartedly that salvation only comes through a person's faith in God. He came to believe this through continuing to read Ephesians 2:8, “For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast" (ESV). He argued that God's great grace saves sinners, in conjunction with the faith that He also provides.
3. Sola Scriptura ("only through the Scriptures alone"): Finally, Luther was a strong advocate of the idea that to understand God's grace and to have faith in Him, the common person must be able to have access to God's Word, the Bible. During a time when only the priests and religious teachers could read the Scriptures, Luther, along with other reformers, believed the common man must be able to hear the Word of God in his own language and understand it for himself. He noted verses like 2 Timothy 3:16 which states, "All Scripture is breathed out by God and is profitable for teaching, reproof, for correction, and for training in righteousness" (ESV). Therefore, since every part of Scripture is God's Word, then the common man must be able to hear and understand it to have faith in God.