The Ottoman Empire was a muslim Empire, and Ataturk sought to "reinvent" the Empire as a modern, European state. He saw the country as in need of radical change, as it was referred to as "Europes sick man" at this time. Ataturk saw it as more beneficial for Turkey to turn towards Europe- at this time at the zenith of her power- for inspiration than to turn towards arabic countries. As European states (especially the elites) had grown more and more secular, and rationalism was going strong, leading the country towards being a secular one was part of this "reform package". As this worked very well for Turkey, the country followed Ataturks vision direction for roughly the next hundred years.
His father was a secularist, so he went to a secular school when he was young. Soon, he saw religion as a hindrance to what he meant was neccessary progress.
He joined the military shortly after high school, and proved himself as a very capable officer. His status became that of a war hero, which also also made him a national hero, giving him enough support among the population to win the 1919 election. He then led the resistance movement against the British after WW1, and contributed strongly to the Turkish victory over the Greeks shortly after, cementing his status as the obvious choise for the position of national leader.