John 1:11-13 “He came to that which was his own, but his own did not receive him. Yet to all who did receive him, to those who believed in his name, he gave the right to become children of God— children born not of natural descent, nor of human decision or a husband’s will, but born of God.”
Romans 8:14-15 “For those who are led by the Spirit of God are the children of God. The Spirit you received does not make you slaves, so that you live in fear again; rather, the Spirit you received brought about your adoption to sonship. And by him we cry, “Abba, Father.” “
Galations 3:26-27 “So in Christ Jesus you are all children of God through faith, for all of you who were baptized into Christ have clothed yourselves with Christ.”
I John 3:2 “Dear friends, now we are children of God, and what we will be has not yet been made known. But we know that when Christ appears, we shall be like him, for we shall see him as he is.”
8 8 Blessed are the clean of heart, for they will see God.
9 Blessed are the peacemakers, for they will be called children of God.
10 Blessed are they who are persecuted for the sake of righteousness, 9 for theirs is the kingdom of heaven. 11
Blessed are you when they insult you and persecute you and utter every kind of evil against you (falsely) because of me.
12 10 Rejoice and be glad, for your reward will be great in heaven. Thus they persecuted the prophets who were before you.
13 11 12 "You are the salt of the earth. But if salt loses its taste, with what can it be seasoned? It is no longer good for anything but to be thrown out and trampled underfoot.
14 You are the light of the world. A city set on a mountain cannot be hidden.
15 Nor do they light a lamp and then put it under a bushel basket; it is set on a lampstand, where it gives light to all in the house.
16 Just so, your light must shine before others, that they may see your good deeds and glorify your heavenly Father.
And so on. Notice the recurring theme here that Jesus includes in every line? Salvation is conditional. Verse 13 is very explicit. When the salt loses its flavour (or the conditions are no longer there), the salt is gotten rid of - or salvation is not given
John 1: the adopted people (ie all men, not just the Jews) were given the power, or the right to become children of God. Not that all of them are. Of course not.
Romans 8: Those who are led by the Spirit are the children of God. Yes. When we no longer are led by the Spirit, but by other fascinations, we are no longer children of God. A condition fully agreeing with the Beatitudes.
Galatians 3: Children of God through faith, clothing yourself in Christ. If you cast off the clothing of Christ, you are no longer a child of God. The last line says that they are heirs to the promise (of God). Heirs don't always collect.
1 John 3: You omitted the first line which says that the Father bestowed love upon us so that we may be called the children of God. Further in the chapter, John also runs through some conditions.
My point? People may refer to themselves as children of God, yet not meet these and other conditions laid upon us by Christ. I cannot say, nor can any man, who is saved and who is not saved, but we have the instructions laid upon us by Christ and those who repudiate them certainly incrase their odds of being repudiated by Christ (Matthew 25). When people say that they "know" that they are saved, this goes against almost continuous message of Christ that God is love, yet salvation is conditional. We must run the race until the very end. Just like the pickers in the vineyard, even those who come late but are accepted by the master will get a full day's wage ie salvation. But if you are not in the vineyard at the end of your day, you will not.