Philip was the son of Landgrave William II of Hesse and his second wife Anna of Mecklenburg-Schwerin. His father died when Philip was five years old, and in 1514 his mother, after a series of struggles with the Estates of Hesse, succeeded in becoming regent on his behalf. The struggles over authority continued, however. To put an end to them, Philip was declared of age in 1518, his actual assumption of power beginning the following year. The power of the Estates had been broken by his mother, but he owed her little else. His education had been very imperfect, and his moral and religious training had been neglected. Despite all this, he developed rapidly as a statesman, and soon began to take steps to increase his personal authority as a ruler.
The first meeting of Philip of Hesse with Martin Luther took place in 1521, at the age of 17, at the Diet of Worms. There he was attracted by Luther's personality, though he had at first little interest in the religious elements of the gathering. Philip embraced Protestantism in 1524 after a personal meeting with the theologian Philipp Melanchthon. He then helped suppress the German Peasants' War by defeating Thomas Müntzer at the Battle of Frankenhausen.
Philip refused to be drawn into the anti-Lutheran league of George, Duke of Saxony, in 1525. By his alliance with John, Elector of Saxony, concluded in Gotha on 27 February 1526, he showed that he was already taking steps to organize a protective alliance of all Protestant princes and powers. At the same time, he united political motives with his religious policy. As early as the spring of 1526, he sought to prevent the election of the Catholic Archduke Ferdinand as Holy Roman Emperor. At the Diet of Speyer in the same year, Philip openly championed the Protestant cause, rendering it possible for Protestant preachers to propagate their views while the Diet was in session, and, like his followers, openly disregarding ordinary Roman Catholic ecclesiastical usages.
Although there was no strong popular movement for Protestantism in Hesse, Philip determined to organize the church there according to Protestant principles. In this he was aided not only by his chancellor, the humanist Johann Feige, and his chaplain, Adam Krafft, but also by the ex-Franciscan François Lambert of Avignon, a staunch enemy of the faith he had left. While the radical policy of Lambert, embodied in the Homberg church order, was abandoned, at least in part, the monasteries and religious foundations were dissolved and their property was applied to charitable and scholastic purposes. The University of Marburg was founded in the summer of 1527 to be, like the University of Wittenberg, a school for Protestant theologians.
Philip's father-in-law George, Duke of Saxony, the bishop of Würzburg, Konrad II von Thungen, and the archbishop of Mainz, Albert III of Brandenburg, were active in agitating against the growth of the Reformation. Their activities, along with other circumstances, including rumors of war, convinced Philip of the existence of a secret league among the Roman Catholic princes. His suspicions were confirmed to his own satisfaction by a forgery given him by an adventurer who had been employed in important missions by George of Saxony, one Otto von Pack. After meeting with Elector John of Saxony in Weimar on 9 March 1528, it was agreed that the Protestant princes should take the offensive in order to protect their territories from invasion and capture.