It is plainly revealed to us, that as many as have believed in Christ Jesus unto the salvation of their souls, are the sons of God. “Beloved, now are we the sons of God.” Here, indeed, is a fat thing.
What, shall a worm of the dust become a child of God? A rebel be adopted into the heavenly family? A condemned criminal not only forgiven, but actually made a child of God? Wonder of wonders! “Behold what manner of love the Father hath bestowed upon us, that we should be called the children of God!” To which of the kings and princes of this earth did he ever say, “Thou art my son”? He has not spoken thus to the great ones and to the mighty, but God hath chosen the base things of this world and things that are despised, yea, and things that are not, and made these to be of the seed royal. The wise and prudent are passed over, but babes receive the revelation of his love.
Lord, whence is this to me? What am I and what is my father’s house, that thou shouldst speak of making me thy child? This gloriously fat thing is also “full of marrow.” There is an inner richness in adoption, for, “if children, then heirs; heirs of God, and joint heirs with Christ; if so be that we suffer with him, that we may be also glorified together.” Well does the apostle remind us that if children, then heirs, for we are thus assured of our blessed heritage. “All things are yours; whether Paul, or Apollos, or Cephas, or the world, or life, or death, or things present or things to come; all are yours; and ye are Christ’s; and Christ is God’s.” “He that spared not his own Son, but delivered him up for us all, how shall he not with him also freely give us all things?” Here are royal dainties of which the Word has said most truly, “They shall be abundantly satisfied with the fatness of thy house.”
Charles Spurgeon, “Good cheer for Christmas,” The Sermons of Charles Spurgeon, Sermons 401-600 (Vol 3)