The reason Unitarians aren't Christian is simple ~ they are not Trinitarians, and some of them are actually Atheists. We can apply the same standards to Jews and we'll get the same result ~ they are not, in fact, Christians.
By necessary implication that standard can be applied to any other group or individual, and you will come up with the same result. Like Buddhists are not Christian. Moslems are not Christian. Jains are not Christian; etc.
Regarding crosses on steeples, due to intense and grinding poverty on the American frontier and newly settled areas (E.g. Western New York), it became quite common for churches to do without steeples as well. Some in the Christian Church Movement still don't have crosses up there ~ but they use the reason that charity must come before building decorations.
Are you seriously proposing that anyone who does not accept the idea of the Trinity cannot be a Christian? By that logic there were no Christians before the Council of Nicaea at the earliest.