Where did this phrase come from? Most use it only in response or association with Jesus, the Christ, who is the first begotten son of God. However, this is a very commony used Hebrew-Jewish term that could cover an assortment of defintions. The most common usage of the title, “son of God“, was one who was deemed righteous. Hosea (1:10) used it as a general reference to all the children of Israel. There were other categories as well.
Isaiah (64:8) cried out to the God who was his and their father. Deut. 14:1 gives the indication that all who are deemed righteous are the sons of God (which would include women, I presume- but probably not back then). 
Moving over to the New Testament, it becomes a little more clear. All those who are “led by the Spirit” are the sons of God (
Since God is love, wouldn’t we be His Sons whether we’re led by the Spirit or not? What loving father says “okay, now you’re my Son as long as you do a certain number of things right or are led by the Spirit a certain percentage of the time?”
Using the prodigal son as a metaphor, perhaps Jesus was like the son who best represented the father by staying back and doing the father’s will all the time while the prodigal son (us) came back not expecting sonship but just a place to exist. The loving father said (in effect) “hey, you were not a perfect representative of me but you’re still every bit as much my son as the the one who stayed behind, represented me and did all I asked!”
I think many of us will be surprised to someday see God putting metaphorical rings on people we disowned but God never did.
Makes sense to me.
But, hard to swallow for the “righteous”.