In the 1500s, God brought about a Reformation of the Christian world: the true gospel was set free from the corrupt Catholic religion and the Word of God was unleashed to save and sanctify once again.  As the movement developed, the Reformers came to agree on five central tenets, which we now call the Solas of the Reformation:

• Solus Christus: Christ alone.  Jesus Christ is the only mediator of salvation between God and man — not the pope, cardinals, bishops, priests, or kings.  Christ alone is the head of His Church, and what He says and does trumps the authority of men.
• Sola Scriptura: Scripture alone.  God's Word alone is the final and highest authority, trumping church tradition and ecclesiological decree.
• Sola Gratia: By grace alone.  Grace alone is the means of salvation.  God is not inclined to save because of anything inherent to man; He saves for His purposes, by His mercy, alone.
• Sola Fide: By faith alone.  Salvation is through faith alone — not through the works or merit of man.  Good deeds contribute nothing to a sinner's justification, and no amount of indulgences (buying certificates to get less time in purgatory) will change that fact. 
• Soli Deo Gloria: To the glory to God alone.  God alone deserves glory in all things.  He alone deserves reverence and worship — not the pope, not the venerable saints, not icons, not relics.