"Service" to God is the performance of righteous, God-pleasing deeds by the Patriarchs, Prophets, Jesus Christ, and Apostles, who gave their pious, sacrificial way of life, devoted to the service of the Will of God.
      "Service" to God is the fulfillment by man of the known Will of God, His Commandments, and the evangelization to others of the necessity of their fulfillment for the true good of all nations.
 Negative Meaning. 
"Service" to "Satan" is the giving of one's time, one's physical and intellectual energies to that which is contrary to the Will of God, His Commandments. It is giving oneself over to the service of political, religious and other idols, cults, ideologies, dogmas, selfishness, one's own sinful impulses of the flesh, to achieve fame, power and wealth.