Bob Dylan once sang, "you're gonna have to serve somebody." It doesn't matter who you are or what you do. Everyone is faced with the fact that they must serve someone. As Dylan puts it, "well it may be the devil or it may be the Lord, but your gonna have to serve somebody."
As Jesus puts it, "No one can serve two masters, for either he will hate one and love the other, or he will be devoted to the one and despise the other. You cannot serve God and money" (Matt. 6:24). Here, the contrast is God or Mammon, but the principle stands for any other thing else as well. Either we will serve God or we will serve something else. People cannot serve God and something else simultaneously, nor can people choose to serve nothing at all. The one who tries this finds that, at very least, he is serving himself and has put himself in the place of God.
The Bible calls anything that people serve, but which is not the one true God, an idol. Anything that becomes a person's central or controlling focus, the thing which a person devotes their time and energies to above all other things, that thing becomes their god. If that controlling focus and chief devotion is not God, that doesn't mean the person is irreligious. Rather it means their religion (formal or otherwise) is idolatry. Everyone worships something. Everyone serves something. Either it will be the true God or it will be a false "god". People are by nature worshipers. It is in our makeup. We were made to worship the one true Creator-God. Many do not worship Him, but that doesn't mean that they are not worshipers. Whether it is the false god of a formal religion or a thing or principle that we've elevated to the place that only God ought to fill in our lives, everyone is a worshiper and everyone is religious.
In his excellent and immensely insightful book, Idols for Destruction: The Conflict of Christian Faith and American Culture, Herbert Schlossberg says this: "...it might be said that a religion should have God as its end. But anyone with a hierarchy of values has placed something at its apex, and whatever that is is the god he serves" (p. 5).
Like Dylan said, you gotta serve somebody.