Also, consider 1 John 4:1-3. Dr Eddie Hyatt, who is very educated in theology as well as a very sincere Christian, said that when these verses were written there was no doubt in the minds of the New Testament believers that what was said meant by "testing the spirits" was to see if they believed in the deity of Christ. This was understood in bible times. The meaning has been lost over the years. Of course, these were not his exact words as I cannot remember them, however, this was the meaning of what he said.
It has proven to be a good test. Just look at all the false religions and cults. They all deny the deity of Christ. There are some borderline groups that do believe in the deity of Christ, but people can and do find salvation in their teachings in spite of bad doctrine. So the borderline groups pretty much have a lot of bad doctrine as I see it. It is doctrine that is so bad that they have been classified as cults even though they believe in the deity of Christ. For example, Oneness Pentecostals and Seventh-day Adventists would fall in those borderline groups.
"Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.
2 Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God:
3 And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
1 John 4:1-3