Lesson of Jesus on Who Belongs to Family of God
Jesus was once told that he had to address his mother and brothers standing outside. This is in Mark 3. Jesus answered that they are not his true family. Then Jesus defined the true family of God. Another time in Luke 11 Jesus was told that his mother who suckeled him as a child was blessed. Jesus answered by explaining in the identical way how a person belongs to the family of God. Both times, Jesus said being a member of God's family meant to hear God's word, and keep it / do the will of God.
Jesus did not use any synonym for believing in God or in himself as a means of belonging to God's family.
If you were asked what does it mean to be a member of God's family, would you answer correctly measured by the words of Jesus? Or would you instead answer with a Pauline grace version -- that it means to believe Jesus is Messiah, ascended into heaven, and rose on the 3d day, and paid for your sins?
But that isn't what makes you a member of God's family according to Jesus, right? Having such beliefs might make you a member of his temporal church on earth. However, such beliefs do not necessarily make you part of God's family -- as long as you measure truth by comparing Jesus's words to Jesus' words.
In Mark 3, we read:
31 His mother and his brothers came, and standing outside, they sent to him, calling him. 32 A multitude was sitting around him, and they told him, “Behold, your mother, your brothers, and your sisters[b] are outside looking for you.”
33 He answered them, “Who are my mother and my brothers?” 34 Looking around at those who sat around him, he said, “Behold, my mother and my brothers! 35 For whoever does the will of God, the same is my brother, and my sister, and mother.” (Mark 3:32-35 WEB)
In Luke 11, Jesus says something similarly that actual familial connection to himself is not the true blessing. Jesus says that instead belonging to God's family brings a blessing which comes by obedience:
27 It came to pass, as he said these things, a certain woman out of the multitude lifted up her voice, and said to him, “Blessed is the womb that bore you, and the breasts which nursed you!”
28 But he said, “On the contrary, blessed are those who hear the word of God, and keep it. (Luke 11:27-28 WEB.)
So we see Jesus identifies those who "hear the word of God and keep it" as the truly blessed and Jesus' true family. It is not simply knowing him as a son of God. Jesus described a true family relationship are those who "do the will of God."
Thus, are we adopted by faith into the family of God? No.
Any verse in Paul that purports to say that is a problem. Either Paul contradicts Christ or Paul is being mis-translated to translate a word in Greek that equally means obedience but is rendered as if Paul used the word to mean faith." Only this latter option avoids having to say Paul contradicts Jesus.
Is that a possible resolution?
The root word PISTIS is translated as faith in many places in the New Testament but its true primary meaning is reliance / trust / faithful / obedience, etc. It actually is not 'faith' at all -- prior to our modern dictionaries adding that meaning.
Why is that?
Because this noun PISTIS was actually the name for centuries of a Spirit in Greek culture who had all these attributes. Pistis was a spirit that was reliable, trustworthy, faithful, obedient, etc. See our article on Pistis. See Pistis from Wikipedia. It was not a credulous / believing spirit.
As a result, even Strong's gives you a secondary meaning of Pistis, besides faith, as "faithfulness" -- which means obedience. See Strong's.
Thus, a shorthand noun to express these concepts was the name of the Spirit - her name was PISTIS in Greek, and FIDES in Latin. The only way to save Paul from contradicting Christ is to change FAITH as the translation, and use what a Greek person would hear when a literary character -- PISTIS -- was invoked by Paul - a noun that personified multiple traits of obedience at once.