If you ask anyone how many persons there are in the Trinity, they will likely tell you there are three. This is obvious- the very word “Trinity” comes from ‘Tri’=three, combined with ‘unity’, meaning three in unity. Three persons, of one essence, as the classical formulation goes.
Sadly though, if we actually take the time to examine how many persons many so-called trinitarians believe in, we will quickly see that there is an extra person afoot. This is because many theologians who have succumbed to the lies of semi-modalism have accepted the Trinity itself as a fourth person. These people take the one essence, or divine nature that is supposed to be shared by the three real persons of the Trinity and imagine it to be a person itself. Another variation of this is to simply imagine that the group of three persons is a single person. By personifying either the group of persons or the divine nature, these false teachers have craftily introduced a fourth person into the Trinity.