Usually the Protestant Christian beliefs are grounded only on the interpretations of the biblical statements. Even in the case of the phrase "son of God" the Bible is the basis. In biblical terminology "son of" could mean in the order of. Ron Rhodes give the following explanation of the term in his book, "Reasoning from the Scriptures with the Jehovah's Witnesses" page 135:
Though the term [son of] can refer to "offspring of" in some contexts,
it actually carries the more important meaning "of the order of." The
phrase is often used that way in the Old Testament. For example, "sons
of the prophets" meant "of the order of prophets" (1Kings.20:35).
"Sons of the singers" meant "of the order of singers"
(Nehemiah.12:28). Likewise, the phrase "Son of God" means "of the
order of God," and represent a claim to undiminished deity.
So the biblical meaning of the phrase "Son of God" in the case of Jesus Christ is "in the order of God." However, it was not used in that sense when it was used in reference to others for they are not the true sons of God like Jesus Christ is.
The phrase "son of God" has been used in the Bible in reference to the following groups...
- Angels [Job.1:6,2:1]
- Adam [Luke.3:38]
- Children of Israel [Deuteronomy.14:1]
- All the believers belonging to the New Covenant [Romans.8:14-17]
- Jesus Christ [Mark.1:1; 1John.4:10]
In the above list, all the groups from 1 to 4 are referred to as merely the "sons of God." Besides, they were all created beings. Whereas only the last group i.e. consisting of Jesus Christ, is given the following specific descriptions that were never employed in reference to others...
- God's "Own" Son [Romans.8:3,31]
- God's "Beloved" Son [Matthew.3:17; 17:5]
- God's "Unique" Son [John.3:16-18]
The above descriptions set Jesus Christ above the rest indicating that He alone is the one who proceeded out of God as it were. In addition to this distinction, all the groups except the last one are alluded to as the created beings in the Bible [Revelation.5:13].
The phrase "The Son of God" is reserved for the promised messiah as the Bible makes it clear [Matthew.16:13-17, 26:63]. Jesus also makes this clear by claiming to be the promised Messiah and equating it with the title "The Son of God" [John.10:36].
Finally, in the light of the Biblical narratives the phrase "The Son of God" conveyed the meaning that the person whoever is the true "The Son of God" is basically equal to God or originated from within God! The following are some of the evidences to support this meaning...
"But He answered them, “My Father is working until now, and I Myself am working. For this reason therefore the Jews were seeking all the more to kill Him, because He not only was breaking the Sabbath, but also was calling God His own Father, making Himself equal with God." (John.5:17-18)
"My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of the Father’s hand. I and the Father are one. The Jews picked up stones again to stone Him. Jesus replied to them, “I showed you many good works from the Father; for which of them are you stoning Me?” The Jews answered Him, “We are not stoning You for a good work, but for blasphemy; and because You, being a man, make Yourself out to be God.”" (John.10:29-33)
In the light of the above evidences we can safely conclude that Jesus Christ is the only true Son of God and everyone else referred to as the sons of God in the Bible are either sons in the metaphorical sense or adopted sense.