and verse 13 makes sure that you realize that it is a possibility if you continue to have the Son.
Tell me where is Judas now?
Like I said ... if you could loose eternal life, its not eternal. You seem to be arguing that someone could have eternal life, then the next minute they dont, then they can have it again, then loose it again.
Since John tells us that he wrote the letter ... so we would know ... perhaps you should read the letter and see what he mentions ...
For example ... he mentions:
If we say we have fellowship with Him, yet walk in darkness
This is how we know we have come to know Him, by keeping His commandments
He who is born of God does not continue in sin
This is how we know that we remain in Him and He in us; He has given assurance to us from His Spirit
etc. etc. Test yourself to see whether you are in the faith.