Heirs Of God
The conception of Jesus Christ (see the Fact Finder question below to understand what really happened around December 25 on the Roman calendar) was by means of The Father, not a human father, but God The Father (see The Holy Father):
"1:18 Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost. 1:19 Then Joseph her husband, being a just man, and not willing to make her a public example, was minded to put her away privily.
1:20 But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost. 1:21 And she shall bring forth a son, and thou shalt call his name Jesus: for he shall save his people from their sins.
1:22 Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying, 1:23 Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.
1:24 Then Joseph being raised from sleep did as the angel of the Lord had bidden him, and took unto him his wife: 1:25 And knew her not till she had brought forth her firstborn son: and he called his name Jesus." (Matthew 1:18-25 KJV)
"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"
Christ is the firstborn child of God:
"8:29 For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that he might be the firstborn among many brethren [see When 'Born Again' Means 'Conceived Again']" (Romans 8:29 KJV)
The Father is The Father because He is creating children of Himself, a fulfillment of the yet-unfinished "1:27 So God created man in his own image, in the image of God created he him; male and female created he them." (Genesis 1:27 KJV)
"8:11 But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you.
8:12 Therefore, brethren, we are debtors, not to the flesh, to live after the flesh.
8:13 For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live. 8:14 For as many as are led by The Spirit Of God, they are the sons of God.
8:15 For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father. 8:16 The Spirit itself beareth witness with our spirit, that we are the children of God: 8:17 And 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.
8:18 For I reckon that the sufferings of this present time are not worthy to be compared with the glory which shall be revealed in us. 8:19 For the earnest expectation of the creature waiteth for the manifestation of the sons of God. 8:20 For the creature was made subject to vanity, not willingly, but by reason of him who hath subjected the same in hope, 8:21 Because the creature itself also shall be delivered from the bondage of corruption into the glorious liberty of the children of God.
8:22 For we know that the whole creation [see Christ The Creator] groaneth and travaileth in pain together until now. 8:23 And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body." (Romans 8:11-23 KJV)
Fact Finder: When was the Messiah conceived? At what time of year was He born?
See What Really Happened On December 25?