Does proclaimed belief in the oneness of God bring a non-Muslim closer to the Ummah?

A person becomes Muslim by showing he believes "There is no god but God and Muhammad is the messenger of God". Given that socio-political custom is a construct of peoples' different geographic and temporal experiences, would the proclamation of the tahlila - showing a belief in Tawheed - bring a non-Muslim any closer to the Ummah? In reverse, if a person of Muslim heritage chooses to only state the first half of the Shahadah, where does he stand?