Welcome to the forum, Dane! (This person actually is from Denmark, btw. Your English is excellent!) Don’t worry if your posts don’t show up immediately at first: all new members go to the spamcatcher automatically for a while, just to be sure they aren’t selling Warcraft gold (is that even still a thing?) or shoes or whatever. After a few more posts the system will click over to letting you through automatically henceforth. (Your post in favor of Licona’s book on the Resurrection also went through this morning. I agree, that book is great. 
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The Bible talks about being a child of God in two ways (or three or four ways, but for salvation purposes two ways) which is one reason why there has been a lot of confusion among Christians over the centuries and millennia. Both ways are important, but the primary way is most important: all rational beings are children of the Father of spirits, upon Whose active and continuing support we rely in order to continue existing at all. (Paul stresses this point even in regard to rebel spiritual powers in Col 1, when he talks about how God, including Christ specifically in the foundational ground of all reality, keeps things in continuing existence, even the spiritual powers which have rebelled against their Creator.) We’re all inheritors of God, even though so long as we are immature we aren’t “enjoying the allotment of the inheritance” as the Greek texts put it (including often in the Gospels). And so long as we continue to sin, we’re still immature. But God intends for us to inherit.
This childhood from God is something that can never be destroyed or taken away. What isvariable, is our chosen relationship to God, which can shift back and forth a lot. In the sense of being followers of God or allies of God, we can be children of God or children of the world (or of the devil etc.) We aren’t always loyal children.
Due to the special problems of rebelling against the ground of our own existence, the only way to come out of our sin relies on God: we must be born from above or born again (the Greek reads both ways at once with the same term), and this which leads to permission of enjoying the full rights of inheritance is what John is talking about in GosJohn 1 (the prologue) when he says that the right to be children comes only from God and not anywhere else, not from nature nor from any lesser persons nor anything else in creation. But that is because our fundamental childhood comes from God in the first place: we already have that when God brings us into being as rational creatures.
So, as long as you have existed as a person, you have in fact been a child of God in that sense, and that won’t ever stop being true. But when you’re being a rebel child, then you aren’t working along with God (including against your sin) and so in a relational sense you are or aren’t a child of God.
Christian churches are generally aware that those who are children of God shall surely be saved from their sins (or sometimes, more bluntly, saved from hell or punishment or whatever). But since they also generally think some rational creatures shall finally be lost one way or another (about which they dispute on the details), the churches are also (logically as far as they go here) interested in trying to ascertain whether someone is finally a child of God or not; whether God has been properly convinced to adopt someone as a child; whether there is sure evidence that God has truly intended for someone to be a child; and so forth. This leads to a lot of variation of the sort you were talking about at the end, looking for one of the two key gospel assurances (while, sadly, denying one of the other ones!) Does God certainly intend to save you, not maybe you? – and will God certainly (not maybe) succeed in saving whomever He intends to save?
Christian universalists (whether Protestant or Western or Eastern Catholic, or whether one of the other ancient trinitarian groups like the Coptics or the Nestorians, or whether one of the non-trinitarian groups – we have a lot of variation 
) will answer yes to both those gospel assurances: yes, God certainly intends to save you from your sins, not maybe you; and yes, God will certainly succeed in saving whomever He intends to save from sin.
Now, we’ll have variances among ourselves about what counts as cooperating with God meanwhile, and how far, and about whether God only heals people sooner or later from their sins (we all agree there’s some healing involved, at least later, generally also sooner to some extent), or else whether God does actively (if sometimes indirectly) disciplines impenitent sinners, and if so whether that’s only in this life or also after death. Since we have a lot of variations on these things, you’ll hear and read a lot of different answers from us, and I’ll let various members take stabs at that! 
You can decide which of us make the most sense on whichever topics.
But hopefully this has been somewhat helpful as a first reply. 