True. I tried convincing this person via email who had a "Christian" website (because he was teaching there that God the Father and Jesus are two separate beings). He just couldn't accept it, even with all the plain scriptures I've provided (the ones below, then many other scriptures I've learned and understood with the Lord's guidance, from Bryan's many studies about the Godhead):
Thou shalt have no other gods before me. (Exodus 20:3 )
Thou shalt have none other gods before me.(Deuteronomy 5:7)
14 for thou shalt worship no other god: for the Lord, whose name is Jealous, is a jealous God:(Exodus 34:14)
2 And, behold, there came a leper and worshipped him, saying, Lord, if thou wilt, thou canst make me clean. 3 And Jesus put forth his hand, and touched him, saying, I will; be thou clean. And immediately his leprosy was cleansed.(Matthew 8:2-3)
I believe that a newly (truly) saved person who hasn't yet studied the "meat" of the Godhead doctrine will still be able to somehow work out with the scriptures above that there are no two Gods.
Though we know what the Lord teaches that lost people are blinded and have hardened hearts, it still amazes me sometimes how they cannot see the most plain of scriptures.