How do we know for certain anyone is and that answer is not specific to any church.
Many say they are Christian but are not born again.
Hum, because they don't do such things as speak in tongues, no spiritual gift manifestation {charismatic} or because something like the preacher didn't baptize them saying in the name of Jesus Christ?
By GOD's Grace through Christ with Faith we are saved and thus Born Again. The contrite heart and acceptance of Jesus Christ allows us to be born again. The moment that happens the very second we surrender to Christ and ask His forgiveness we are born again and our soul Baptized in His Blood. The person may show no outward or even at that time feel any inward emotion. That for some comes later.
Churches make salvation so complicated yet Christ made it so a child could come to Him. A child does not have to know Latin, even know The Bible for that matter when salvation comes.
I was hounded once for about two years by a Charismatic Zealot who insisted I was not born again, saved, etc simply because the preacher did not specifically say "In the name of Jesus Christ". My faith at that time in my life and my knowledge had not matured. Yet I was still saved. The knowledge and growth would come about seven years later in GOD's appointed time in my life.
I don't believe being a Catholic makes anyone lost or saved anymore than I would believe being a Protestant from any sect makes one lost or saved. It's not about the church. It's not the name of the church or the church that brings one salvation. They are buildings for groupps or people to worship. It is whether the individuals within them or elsewhere for that matter accept or reject Jesus Christ. I believe all churches every songle one known to man are in some way imperfect in doctrine but the believers though they err in dogma, doctrine, or theological issues are saved by Grace.
All sin can be forgiven except the persons final rejection of Jesus Christ. That is a persons harden heart where they ignore or reject The Holy Spirits conviction and call of salvation to where their heart is hardened toward GOD.
My point all along has been that once one is born again one will leave a church organization, such as Catholicism, that teaches so much error and unscriptural doctrine.
How does a born again believer filled with the Holy Spirit take mass or this Eucharist and justify it? That is where we part company. No one, born again of the Spirit, stays in a false religion.
I have heard some say though that there are born again Mormons and Catholics who stay where they are out of fear of losing family and friends. If that is the case, then I don’t believe they were ever born again and Spirit filled.
The Holy Spirit is living in us precisely to lead us out of error and fear and into truth, so there can be no excuse to stay. One’s comfort, and friends, and family are not above Chrsit and we are called to leave them behind if they hinder our walk with Him.