I'm interested to learn how a believer may be illegitimately saved?
The word "believer" in my post had quotation marks for a reason yet you omitted them. I'm going to operate as if you didn't catch that or recognize the significance. Its a distinction with a difference i point out as needed.
Satan and the demons "believe" ( in fact they know beyond any shadow of doubt) Jesus is Lord yet they are not "saved".
There are many false doctrines out there by false (Christs, Apostles, prophets, churches and so forth) which have a false salvation.
One of the reasons Scripture is the authority is because of Satan and man twisting the truth because 99% of the truth is a lie. 99 truths ( in and of themselves) out to get you to believe #100 which is the coup de gras is also a lie.
So, there is a class of person who has a legitimate belief that they are 'saved" ( and possibly even with Satan and his minions giving the "emotional support and warm fuzzies" to make it look legit) but in fact are damned right along with the rest of them. ( Those who "believed' and did great works in His name and all that- who Jesus made clear "I
NEVER knew you")
There is 1 truth and a million gazillion ways for Satan to deceive- all of them end in a lake of fire.
So, keep right on doing things your way and twisting scripture to support your "philosophy" to your own personal desires rather than studying and understanding the truth and meaning then yielding to the Word of God as a good and faithful servant would- you will have your reward. (it just might not be what you think)
Carry on.