You cannot then call yourself an atheist or unbeliever. Life itself is shallow. Being itself is surface only. He who knows about depth knows about God. When the word ultimate concern crops up, Tillich has often been accused of making God too subjective, too relative.
Ultimate concern is crucial to genuine god-talk.
But Tillich is not saying that having an ultimate concern is all we need. Whatever concerns us ultimately may be a god for us, but it may be a false god, an idol. If we are ultimately concerned about these they become gods for us. But they are not God. They are idols.