The Apostle Peter told us that many false religious teachers will come from within the church itself.
2 Peter 2:1 (NASB)
But false prophets also arose among the people, just as there will
also be false teachers among you, who will secretly introduce
destructive heresies, even denying the Master who bought them,
bringing swift destruction upon themselves.
On the other hand, it is said by Jesus Christ and his disciples that the church would never be totally apostatized.
Matthew 16:18 (NASB)
18 I also say to you that you are Peter, and upon this rock I will
build My church; and the gates of Hades will not overpower it.
1 Timothy 4:1 (NASB)
But the Spirit explicitly says that in later times some will fall
away from the faith, paying attention to deceitful spirits and
doctrines of demons,
This means that over the span of 1400 years ( From the 2nd century up to the 14th century)there were genuine born again Christians.
Romans 11:1-5 (NASB)
I say then, God has not rejected His people, has He? May it never be!
For I too am an Israelite, a descendant of Abraham, of the tribe of
Benjamin. 2 God has not rejected His people whom He foreknew. Or do
you not know what the Scripture says in the passage about Elijah, how
he pleads with God against Israel? 3 “Lord, they have killed Your
prophets, they have torn down Your altars, and I alone am left, and
they are seeking my life.” 4 But what is the divine response to him?
“I have kept for Myself seven thousand men who have not bowed the knee to Baal.” 5 In the same way then, there has also come to be
at the present time a remnant according to God’s gracious choice. 6
But if it is by grace, it is no longer on the basis of works,
otherwise grace is no longer grace.
A born again Christian is someone who placed his/her 100% complete trust/confidence (faith) on Christ as His God and Savior. They became born again through the Holy Spirit.
Romans 1:16-17 (NASB)
16 For I am not ashamed of the gospel, for it is the power of God for
salvation to everyone who believes, to the Jew first and also to the
Greek. 17 For in it the righteousness of God is revealed from faith to
faith; as it is written, “But the righteous man shall live by faith.”
Romans 6:23 (NASB)
23 For the wages of sin is death, but the free gift of God is eternal
life in Christ Jesus our Lord.
Romans 10:9-10 (NASB)
9 that if you confess with your mouth Jesus as Lord, and believe in
your heart that God raised Him from the dead, you will be saved; 10
for with the heart a person believes, resulting in righteousness, and
with the mouth he confesses, resulting in salvation.
Titus 3:5 (NASB)
5 He saved us, not on the basis of deeds which we have done in
righteousness, but according to His mercy, by the washing of
regeneration and renewing by the Holy Spirit,