Thomas Aquinas (1225-1274) represents a theological legacy of the Medieval Times with his Scholastic theology that covers most of the matters comprehensively and systematically. Then, do his enormous works contain any portion of writings that address the missional concerns of his time? He lived his life in the midst of the Crusades when the relationship between Christians and non-Christians in general got worsened and the tension between Christians and Muslims in particular got more aggravated. Among his works, this study inquires about three magna opera, i.e, Summa Theologica, Summa Contra Gentiles, and De Unitate Intellectus Contra Averroistas. Each demonstrates the quintessence of his theological and philosophical genius and stance. In what sense do these three works contain Tomas’s missional concerns for so-called the Infidels of his time? Who are they? What are the missional implications of his works? Thomas laid the groundwork of his approach and outreach to the Infidels. For muslims who do not accept the authority of the Christian Scripture, it is not helpful to use the Bible. They don’t have the capacity to understand the divine truth based on the special revelation of God. Rather, he uses the natural reasons to refute the errors of pagan thoughts based on Aristotelian philosophy. Aristotelianism was warmly welcomed in the Islamic world and flourished during the Middle Ages. For Jews, he argues, Christians can use the Old Testament to refute their errors in Judaism. Likewise, the New Testament can be used to correct the heretics and apostates who were once Christians. Overall, Thomas Aquinas played his role as a philosopher rather than a theologian to communicate the Christian message to the unbelievers, especially to the muslims. In this sense, he was a missional apologist of his time.