The Jehovah Witness' are not a Cult in the traditional sense of the word, they are a Restorationist Christian Denomination. However it can be argued that many restorationist denominations also fit many of the social criteria of a Cult, other denominations that would also be considered thus are the Christadelphian, Latter Day Saints and The Brethren.
They are generally rejected by mainstream Christianity, with most mainstream and liberal Christian Movements rejecting their interpretations as being Christian at all, particularly the Jehovah's Witnesses as they have altered the Bible without the required justification, either theologically or more importantly lingustically.
As for the OP, you do not need organised religion to hold a belief in God, and if we look at Christianity in particular, Jesus did not create a religion, but preached a ministry....one which it can be argued is opposed to organised religion insofar that religion seeks to bind its adherents to God in a common path, whereas Christ taught that it was up to the individual to find their own path to God. This is one of the basic ideas behind Evangelism (and I don't mean the conservative kind often seen in the US) and formed the basis of the anti-clerical movement amongst Christians. Essentially Religion is a social and cultural manifestation rather than a spiritual one, you do not have to belong to an organised Church to be a Christian for example.