Theism is the general belief in a god. It is about the broadest possible category of religion possible. Whether one is a Christian, Muslim, Jew polytheist, pantheist, deist, or virtually any other religion, he is a deist. It does not matter what this god is like, if he is loving or cruel, personal or distant, one or many. It is just believing in the existence of any higher power. The only religious beliefs not included in this label are atheists and agnostics.
Believing in a god is obviously a requirement to believing in the God. However, it's not really a Christian doctrine, as there is nothing in it that is specific to Christianity. It isn't righteous to just believe in a god. The Bible is clear you have to believe in the one, true God:
Exodus 20:3 Thou shalt have no other gods before me.
John 14:6 Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me.
More than that, it is not enough to just believe in Jehovah God either.
James 2:19 Thou believest that there is one God; thou doest well: the devils also believe, and tremble.
Even Satan believes in God. Theism alone cannot save you. The only way to be saved is to go a step further and believe on Christ, repent of your sins, and trust in God alone to save you.