A principle found in the Bible makes this clear. We read: “Do you not know that if you keep presenting yourselves to anyone as slaves to obey him, you are slaves of him because you obey him, either of sin with death in view or of obedience with righteousness in view?” (Rom. 6:16; Matt. 6:24) Thus, when a person goes contrary to God’s will, he pleases God’s adversary. Whether the person believes in Satan or not, by conforming to the world and its unrighteous ways, he serves the interests of Satan.

The Jews in the first century C.E. were acquainted with the Hebrew Scriptures, as these were read every sabbath in the synagogues. (Luke 4:17-21; Acts 15:21) But they failed to act in harmony with those Scriptures and rejected the evidence in them that pointed to Jesus as the promised Messiah. They refused to accept the truth as revealed through Jesus. By choice, their knowledge of God’s will remained incomplete.

As a consequence, many Jews made the Devil their god. How did they do this? Not by their saying that they served the Devil, but by reflecting a murderous disposition toward Jesus. That is why Jesus plainly told them: “If God were your Father, you would love me, for from God I came forth and am here. Neither have I come of my own initiative at all, but that One sent me forth. Why is it you do not know what I am speaking? Because you cannot listen to my word. You are from your father the Devil, and you wish to do the desires of your father. That one was a manslayer when he began, and he did not stand fast in the truth, because truth is not in him. When he speaks the lie, he speaks according to his own disposition, because he is a liar and the father of the lie.”​—John 8:42-44.

It means that, to please God, you must accept all of God’s Word and conscientiously apply it to every aspect of your life.

Is a person doing that if he indulges in superstitions or pagan religious rites while claiming to be a Christian? In various parts of the earth people who claim to be Christians also consult astrologers or fortune-tellers, even though the Bible condemns this. (Deut. 18:10-12) They may also employ the services of a “medicine man” in an attempt to be cured of bodily ills. They see nothing wrong in the fact that various gods are invoked in the ritual. Are such persons serving the true God? How could they be when his Word says: “What fellowship do righteousness and lawlessness have?”​—2 Cor. 6:14.

This was the very thing of which unfaithful Israelites became guilty. They claimed to serve Jehovah and at the same time sacrificed to other gods. They saw nothing wrong in mixing idolatrous practices with their worship of Jehovah. (Jer. 44:16-19) But did Jehovah approve of their worship? No. The prophet Jeremiah told them: “As for the sacrificial smoke that you made in the cities of Judah and in the streets of Jerusalem, you and your forefathers, your kings and your princes and the people of the land, was it not this that Jehovah remembered and that proceeded to come up into his heart? Finally Jehovah was no longer able to put up with it because of the badness of your dealings, because of the detestable things that you had done, and so your land came to be a devastated place and an object of astonishment and a malediction, without an inhabitant, as at this day. Because of the fact that you made sacrificial smoke and that you sinned against Jehovah and did not obey the voice of Jehovah and in his law and in his statutes and in his reminders you did not walk, that is why there has befallen you this calamity.”​—Jer. 44:21-23.

Jehovah God does not change. (Mal. 3:6) All those who are not exclusively devoted to true worship face calamity. Why? Since they are not on Jehovah’s side, they are servants of Satan the Devil and his wicked demons. The apostle Paul wrote to Christians at Corinth: “The things which the nations sacrifice they sacrifice to demons, and not to God.”​—1 Cor. 10:20.

Similarly, even though a person claims to worship the true God, if he treats created things as objects of reverence, this is a serious matter. This was recognized by faithful Job when he observed: “If I used to see the light when it would flash forth, or the precious moon walking along, and my heart began to be enticed in secrecy and my hand proceeded to kiss my mouth, that too would be an error for attention by the justices, for I should have denied the true God above.” (Job 31:26-28) So, if a person used images as an aid in worship or began idolizing a person or thing, would he be serving the true God? The Bible answers: “I am Jehovah. That is my name; and to no one else shall I give my own glory, neither my praise to graven images.” (Isa. 42:8) So, then, if by his thoughts or actions a worshiper “denied the true God,” to whom would his worship go? Who would be his god? Obviously, “the god of this system of things,” the Devil.