Judeo-Christian acknowledgment of the Messiah understood God's providencial relationship with the Jews and the Gentile times via Christ. Those who reject God are simply unregenerate.
Present luke-warm definitions of religion which allow any form of spiritism, or belief can also be taken to an extreme. One could respect a 'religion' based entirely upon criminality. Such a freedom is self-contradictory.
The main reason we colloquially accept beliefs other than Judeo-Christianity, IMHO, is that most people simply don't study Scripture and associate anything dealing with the spiritual domain or belief of supernatural beings as qualification of 'religion'.
By no means did the founding fathers interpret the Islamic, "Convert or be killed" policy as a comparable 'religion'.