Matthew 12.50 promises, whoever does the will of My Father in heaven is My brother and sister and mother.
It is interesting to note what God DID NOT say made people His brother and sister and mother. His relations are not among those who believe in God but is among those who obey God.
Jesus, as God, did not say everyone who believes that there is a God or that God is good, or that God loves everyone or anything else people believe about God, make people His relatives. Certainly Jesus believed in God since He is God.
God proclaimed that He has had a constant enemy. This enemy fought Him the moment He started His public ministry. The enemy worked among His twelve closest friends in order to cause one of them to betray Him. It is this enemy that believes in God but is no friend of God which is why believers are not part of the family of God.
Almost everyone believes in the concept of god. There are very few pure atheists in the world. When times are immensely tough there are even fewer atheists. This is because the One True God has placed in the heart of His created beings, a homing device that causes us to cry out to Him though we are ignorant of Him and have spent most of our lives ignoring Him.
The true family member of God in the flesh, the true children of God are all those but only those who obey God’s commands as given by God through the Son Jesus Who is the Christ. What we believe or what we say means nothing, it is what we do. If we love God we will obey God.
We may believe we love God but if our actions demonstrate otherwise we deceive ourselves and the truth is not in us. God will measure our deeds not our intentions. Our words too will be used for us and against us on judgement day but in conjunction with their accompanying actions. If we say we love God but hate our neighbor we deceive ourselves again and the truth is still not in us.
God the Father expects His children to live as God the Son lived. God the Son gave His life as a ransom so that some could be reconciled to God and live eternally with Him.
Those who engage the will of God by doing the work of God which is the restoration of the world to Him through Christ will enjoy hearing their eternal Father one day say, well done good and faithful servant, enter into the joy of your Master.