"Do not bow down to them"Exodus 20:5.

It is forbidden to worship any deity other than the One G‑d in any of the following manners: 1) Bowing. 2) Slaughtering a sacrifice. 3) Pouring a libation. 4) Burning a sacrifice on an altar. These prohibitions apply even if these acts are not the accepted way of worshipping that particular deity.

Aside for these four actions, it is also forbidden to verbally declare belief in a deity other than G‑d.