An honest response. But why is it a litmus test for an entire sect of Christianity must be imposed from the outside for them to be “real” Christians?
It's not a litmust test for one sect - it's a litmus test for everybody. If a Baptist hasn't been born again, they're not a Christian. If a Presbyterian hasn't been born again, they're not a Christian. If a Catholic hasn't been born again, then they're not a Christian. It applies to everybody, across the board.
Because that's what the Bible says - and GOD gets to define who is a Christian and who isn't.
Jesus said, "Except a man be born again, he cannot see the kingdom of God." (John 3:3) That's a litmus test. Jesus is saying that except a person has one, particular event happen in their life, that person won't see the kingdom of heaven. All this wishy-washy "well, you just can't ever know" stuff doesn't fly when viewed from the perspective Jesus sets here.
God says that repentance is necessary for salvation, "Repent ye therefore, and be converted, that your sins may be blotted out...." (Acts 3:19). Without repentance, without the specific choice in the heart and mind of a person to turn away from their sin and desire to live for God instead, sin cannot be blotted out. Repentance is really just a part of faith - a person who won't repent is a person who cannot credibly make a claim to having placed faith in Christ.
God tells us that a person has to believe and call upon God for salvation through Jesus Christ. "That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved....For whosoever shall call upon the name of the Lord shall be saved." (Romans 10:9,13)
God also tells us that this is completely apart from any works which we might think to do to "earn" salvation. "Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost, Which he shed on us abundantly through Jesus Christ our Saviour." (Titus 3:5-6)
THESE are the criteria for Christianity - not what denomination one holds to, etc.