Martin Luther contributed much to both the Renaissance and early Modernity. One of his chief contributions was the translation of the New Testament into German. Historically, the Catholic Church had opposed to the translation of Scripture into the common or "vulgar" languages, because they believed only the priests (as instructed by the Church) could accurately interpret what the Bible meant. Luther argued for the "priesthood of all believers," the idea that all Christians could directly communicate with God and thus had the ability--through the help of the Holy Spirit--to interpret Scripture alongside the Church.
The proliferation of translations of Scripture increased the need and desire for public literacy (the ability to read and write), and Luther became a strong proponent of the creation of public Christian schools that would teach pupils to read the Bible. This coincided well with the Renaissance movement for educational reform.
Martin Luther (1483-1546) was an unwitting religious reformer. In other words, he did not set out to start what would be known as the Protestant Reformation, but that is exactly what he did. So, in short, Luther's contribution to the Renaissance was to create a religious movement and give birth to Protestantism.
Luther was able to accomplish this, because during the Renaissance people questioned things, thought critically, and were open to new ideas. So, when Luther saw the unjust practices of the Catholic church, such as the selling of indulgences, he called the church to reform its way. By his famous 95 theses, which he put on the castle door of Wittenberg, he threw down the gauntlet. Luther asserted that his conscience was captive to the Word of God and that he could not go against his conscience.
His contribution, therefore, was the ability of people to question religious authority and follow their conscience.