We all refer to our human fathers as “father,” “dad,” etc., but in this case, Jesus was referring to our spiritual Father.

In the Old Testament, people generally related to God as The God, a judge, a friend, or a husband (to the nation of Israel), but rarely as their Father. So, it was a wonderful blessing when Jesus encouraged the tender relationship of Father and child between God and man. Jesus was about to ransom men from sin and death. Therefore, through belief of Jesus’ sacrifice, individuals could be adopted into God’s family. Romans 8:15, “…you received the spirit of adoption, whereby we cry, Abba, Father.” (ASV)

As Jesus was preparing to ascend to his Father, He reassured Mary in John 20:17, “Do not hold onto me, for I have not yet returned to the Father.  Go instead to my brothers and tell them, I am returning to my Father and your Father, to my God and your God.” (NIV) However, INDIVIDUALS were not referred to as having a Father/child relationship other than Adam before he sinned and Jesus at his first Advent. You were either a friend, an enemy or a servant of God during these intervening years. 

At the tail end of the genealogy of Jesus, there is a significant statement made: Luke 3:37-38: …the son of Methuselah, the son of Enoch, the son of Jared, the son of Mahalaleel, the son of Cainan, the son of Enosh, the son of Seth, the son of Adam, the son of God (Greek: Theos).

Adam, the perfect man created in God’s image, was “the son of Theos.” Adam was considered a son before he sinned. Thousands of years would go by before the relationship of being a son to God would again exist. Once sin entered, sonship was set aside and being a friend of God or a servant of God became the closest any human could come until the subtle statement made by the angel Gabriel to Mary: Luke 1:34-35: 34 Mary said to the angel, How can this be, since I am a virgin? 35 The angel answered and said to her, The holy spirit will come upon you, and the power of the Most High will overshadow you; and for that reason the holy child shall be called the son of God (Theos).

Additional Resources: 
Christian Questions Podcast 3-Part Series: “Who is God?”
(The Father/son relationship is a specifically discussed in Part III)