The holy mountain is considered the Temple. They also have a "holy rock" on it where they pray. Their schism with the exiled Jews goes back to the 5th century BC. If a Samaritan wants to "become" a Jew, he must renounce his belief in the holiness of Mt. Gerizim.
This fact makes John 4:20 a lot clearer. The Samartian woman tells Jesus:
Judaism treats Samartians as Jews where their beliefs and practcies coincide with thsoe of mainstream Judaism.