As contributors before me have already pointed out, the two key influences to the Renaissance were Ancient Greece and Ancient Rome.
However, just as a point of clarification, I would argue that the Middle Ages were not quite as hostile towards Greek and Roman antiquity as some answers here might have suggested. For one thing, it should be noted that Christianity came out of the Roman Empire, so whether we're talking about the early Christian martyrs or the Church Fathers, we're still actually talking about a Roman context. In addition, when speaking about Christian philosophy and Scholasticism, one can point towards the widespread influence which Neo-Platonism and (later) Aristotlianism had upon it. Thus, one can actually find Greek and Roman influences on this earlier era as well, though it was an influence of a very different kind than what you would find in the Renaissance.
The Humanists were very much focused on reviving older, classical ideas of civic virtue. I would also say that...
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