I think it's really important for God's people to bear in mind that there are true born again saints spread throughout many denominations. It's not right to say or assert that someone is not truly saved just, because they are attending a certain place( even if belongs to a denomination that has a bad reputation or is different from their own). Now I believe God would lead every one of his true children to leave a place that embraces apostasy, the indwelling holy spirit would prompt them out, but then again I think it's one of those matters we can't say is black and white. The truth is that sadly many churches don't make disciples and have strong teaching anymore. It's created a very suspicious landscape for the elect. Ten years ago I attended a baptist king james only church. We did allot of things right, but they held some beliefs I didn't fully agree with, but they were one of the only few churches that would go into the streets with a burden to witness and I loved that, because I myself was burdened with the same. God gave us great joy as we went out and did street evangelism. However, the thing that never fully sat right with me was they were KJV only. Not only that, but they only really accepted baptist churches as legitimate and the pastor contended if you got baptized in another denomination it didn't count. They were suspicious of everyone who wasn't a Baptist.
I have reformed theology friends who basically say anyone who doesn't hold a calvinist worldview doesn't understand salvation properly and thus isn't saved. I know there are many churches that teach if you don't pray in tongues your not saved. There's so much division nowadays and so many churches claim they are the only way, orthodox churches also hold that view.
The truth is I think it really comes down to whether or not someone has the testimony of Jesus within themselves. If someone has the witness within themselves and confirms to you that they have in fact received Christ(as the only savior) and believe he is the son of God, that he died for their sin and rose again on the third day, that he is the only way to heaven, that nobody has any merit or righteousness before God apart from him, that The Bible is the absolute and inerrant word of God and every other religion is a false way, then such a person has the testimony of Christ within themselves. Nobody could be fully persuaded and convinced of these things unless The Father revealed it to them. Nobody could say that Christ is Lord and believe in these things with absolute conviction without the spirit.
I go to a prayer gathering every thursday and not everyone goes to the same church and denomination, the place we meet consists of like 3 or 4 types, but the people who do go have the testimony of Jesus within themselves(that is obvious) and they all know the heart of God, they are all burdened for souls to come to know Christ, they all know Jesus is this world's only hope. Nobody would be consistently in that room unless God drew them there!
We are told to try the spirits. If someone has the testimony of Jesus within themselves and holds to God's word and believes it's all true then it's my belief they are a true son or daughter of God. Guaging whether or not someone is a true child of God becomes a very slippery slope when its about something other than whether or not they have the testimony of Jesus, because every born again Christian is in a different season in their life.
1 Corinthians 12:3
New Living Translation
3 So I want you to know that no one speaking by the Spirit of God will curse Jesus, and NO ONE CAN SAY JESUS IS LORD, EXCEPT BY THE HOLY SPIRIT.
1 John 5:10-12
English Standard Version
10 WHOEVER BELIEVES IN THE SON OF GOD HAS THE TESTIMONY IN HIMSELF. Whoever does not believe God has made him a liar, because he has not believed in the testimony that God has borne concerning his Son. 11 And this is the testimony, that God gave us eternal life, and this life is in his Son. 12 Whoever has the Son has life; whoever does not have the Son of God does not have life.
Matthew 16:13-17
English Standard Version
Peter Confesses Jesus as the Christ
13 Now when Jesus came into the district of Caesarea Philippi, he asked his disciples, “Who do people say that the Son of Man is?” 14 And they said, “Some say John the Baptist, others say Elijah, and others Jeremiah or one of the prophets.” 15 He said to them, “But who do you say that I am?” 16 Simon Peter replied, “YOU ARE THE CHRIST, the Son of the living God.” 17 And Jesus answered him, “Blessed are you, Simon Bar-Jonah! For flesh and blood has not revealed this to you, BUT MY FATHER WHO IS IN HEAVEN.
Ephesians 1:13-14
English Standard Version
13 In him you also, WHEN you heard the word of truth, the gospel of your salvation, AND BELIEVED in him, WERE SEALED with the promised Holy Spirit, 14 WHO IS THE GUARANTEE[a] of our inheritance until we acquire possession of it, to the praise of his glory.
Romans 8:9-11
English Standard Version
9 You, however, are not in the flesh but in the Spirit, if in fact the Spirit of God dwells in you. Anyone who does not have the Spirit of Christ does not belong to him. 10 But if Christ is in you, although the body is dead because of sin, the Spirit is life because of righteousness. 11 If THE SPIRIT OF HIM WHO RAISED JESUS FROM THE DEAD DWELLS IN YOU, HE WHO RAISED CHRIST JESUS[A] FROM THE DEAD WILL ALSO GIVE LIFE TO YOUR MORTAL BODIES through HIS SPIRIT WHO DWELLS IN YOU.