You said "BTW-There are over 2 billion "professing" Christians out there and, I would take a guess that most are false."
So how can most of 2 billion Christian's have it so wrong? How do you or any other believer make a faith check just in case.
MANY Christians do NOT "walk in the Light as He is in the light" They hide their sin and do , say or act in different way when around the church than they would when not being observed by the other congregants. Many take His grace for granted thinking, "oh, He won't mind this on little sin"
God does not wink at sin, especially since He sent His son Jesus to pay the price of it.
So many think just showing up at a building will save them, ha...good luck with that!
How to know one is not always easy, unless you have your senses trained through the word:
"We do this by studying and knowing intimately God’s Word. We learn from the Parable of the Weeds in
Matthew 13:24–30 that Satan works to deceive the church by mingling his children with God’s children, often making it difficult for believers to discern the true from the false. The more we are familiar with Scripture, the easier it will be for us to detect the true Christians from the false converts."
Satan is in every church, for sure. But, God say's to let the wheat AND the tares grow together until harvest time when He will separate the sheep from the goats.
Some might not even realize they are not converted.
The more we read scripture, the easier it is to detect a false convert. The Spirit gives us discernment, and the more of the word we know, the more apparent those ones are and we are told to avoid them.
Many will fool others though...have seen it.