Romans 8:15 For you did not receive the spirit of slavery to fall back into fear, but you have received the spirit of sonship.  When we cry, "Abba! Father!"  God is the Father of all who believe in Christ in a special sense not shared by unbelievers.  God is called their Father, first of all, because they have a new standing before Him.  While unbelieers are the offspring of God because He created them (Acts 17:28, 29  for 'In him we live and move and have our being'; as even some of your poets have said, "For we are indeed his offspring.'  Being then God's offspring, we ought not to think that the Diety is like gold, or silver, or stone, a representation by the art and imagination of man.), they do not have the standing of sons.  Their standing is rather as condemned sinners before God the Judge (John 3:18 He who believes in him is not condemned; he who does not believe is condemned already, because he has not believed in the name of the only Son of God.; Rev. 20:11 Then I saw a great white throne and him who sat upon it; from his presence earth and sky fled away, and no place was found for them).  When a person believes in Christ as Saviour, his estate is wonderfully changed from grim condemnation to priviledged sonship.  This new standing grants to all believers the legal right and spiritual privileges of divine sonship: "heirs of God, and joint-heirs with Christ" (Romans 8:17 and if children, then heirs, heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him).

God is the Father of believers also in the sense that He gives them new life (John 3:3 Jesus answered him, "Truly, truly, I say to you, unless one is born anew, he cannot see the kingdom of God.").  This relationship then is a family one involving many of the same realities that exist between an earthly father and child: birth of the child (John 3:3); partaking of the father's nature (2 Peter 1:4 by which he has granted to us his precious and very great promises, that through these you may escape from corruption that is in the world because of passion, and become partakers of the divine nature.); the father's care of the child (Matthew 6:32, 33 For the Gentiles sesek all these things; and your heavenly Father knows that you need them all.  But seek first his kingdom and his righteousness, and all these things shall be yours as well.; 7:9-11 Or what man of you, if his son asks him for bread, will give him a stone?  Or if he asks for a fish, will give him a serpent? If you then, who are evil, know how to give good gifts to your children, how much more will your Father who is in heaven give good things to those who ask him!); and the father's discipline of the child (Hebrew 12:6-8 For the Lord disciplines him whom he loves, and chastises every son whom he receives.  It is for discipline that you have to endure.  God is treating you as sons; for what son is there whom his father does not discipline?  If you are left without discipline, in which all have participated, then you are illegitimate children and not sons).  Furthermore, this new Father-child relationship carries with it new brothers and sisters (Hebrew13:1 Let brotherly love continue).

To obtain God as Father is not a result of one's own merit but a result of Christ's.  The one who believes in Christ as Savior enters into the blessed Father-child relationship with God solely on the grounds of Christ's sonship (Romans 8:17 and if children, then heirs, heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him.; Hebrew 2:17 Therefore he had to be made like his brethern in every respect, so that he might become a merciful and faithful high priest in the service of God, to make expiation for the sins of the people.).  It is the grand privilege and calling of those who know God as Father to graciously invite unbelievers to meet God as Father and not as Judge.