Look, we have the doctrine of the early church. It's laid out clearly in scripture. Compare the doctrine of Rome and the doctrine of a faithful Protestant church to the New Testament, and there's no contest.
But that's where I'm going with this post (there are tons of resources already available for that - a few seconds on Google will find you something far better than I could write). Instead of that, I thought it would be fun to see how far back I can trace the distinctive doctrines of the Reformation, and frankly it's not possible to go back much farther. Specifically, I want to show that the five solas are all present (to some extent) in Genesis 1:1. Here's the verse in case you're unfamiliar:
"In the beginning, God created the heavens and the earth".
God is the creator, and everything else is His creation. God exists; everything in the entire created universe exists contingently, at God's pleasure. We came into existence at God's command, and we remain in existence only through His continued will. Everything - the air we breathe, the food we eat, the planet we live on, life itself - is a gift of God. He provides everything, we contribute nothing. This is clearly sola gratia - grace alone. Every aspect of our very existence is a gift of God, not just our salvation. This was true of Adam pre-fall, and it's true for the mightiest angel in heaven. How much moreso must it be true for the salvation of rebellious fallen man? If our entire being is by grace alone, how can our salvation not be sola gratia as well?
Now consider the relationship between Creator and the created. The Creator supplies everything for His creation; the creation provides absolutely zero for the Creator. What can we give to God that was not already His? What can we possibly do for God to earn His beneficence or put Him in our debt? Here is the complete list of what God needs from us: ....and, we're done. Zilch, nada, bupkis. Please, tell me how we can merit anything from God by our awexome works of righteousness. Thought so. The creator/creation distinction necessarily brings sola fide, faith alone. We can do nothing for God, we can only faithfully trust His provision for us. And again, this is true of all of creation all the time; how much more must it be true for rebellious sinful creatures? What could we ever do to earn anything from God, especially after we have committed cosmic treason?
God is the omnipotent provider of all, and we are entirely dependent upon Him for life and breath and everything else. The interaction between us is entirely one direction; God gives us everything, and we give Him nothing. Our proper orientation towards our Creator is perpetual complete worship; He is worthy of all glory and honor and praise. What could we ever do to deserve glory from the one who created and sustains our entire existence? Clearly, this is soli Deo Gloria, and again this would be true even if we had never fallen. The idea that we, as sinful creatures, deserve any glory for our salvation is beyond absurd.
Now imagine if one of the created beings was to rebel against his creator, exalting himself over God. Insane, I know, but bear with me. What would it take to mend this relationship, to turn aside the wrath that God would rightly have against such insolence? Is there anything the creature could do to make things right? Any gift or sacrifice he could offer is something God made and already owns. What would be an appropriate expression of reconciliation from the one who rebelled against the sovereign creator of all? There is nothing in all creation that a creature can give to appease the Creator. No, any reconciliation would necessarily be initiated by God, who alone would bear any cost himself. This of course points us to solus Christus; though the details would come later, we can see immediately that the reconciliation of God and sinful creatures must be of divine origin.
Finally, we can ask: what would a creature know of its Creator, and how would he learn it? Could he by observation and deduction figure out what God is like, and what God demands of him? By looking at creation, what can he know about one who is not part of that creation? Simply put, a man can only know about God what God chooses to reveal about Himself. This is the beginning of sola scriptura, but of course it doesn't get us there. From this verse alone we can get to 'revelation alone' (sola revelata?), but the exact form of that revelation (general vs special, scripture only or scripture plus papal imaginings) needs to be fleshed out later.
There you have it. Three are necessary just from the creator/creature distinction, the fourth is there in all but name, and the fifth just needs to be fleshed out. Far from being inventions of the 16th century, these doctrines are rooted before time began.