With the possible exceptions of Roman 9.5, John 1.1, & Thomas' proclamation "my Lord and my God" in John, the early church used "God" to designate 'God the Father,' and "Lord" to designate 'God the Son'/Christ. In this context, Lord still means God, and corresponds to the same names used of God in the OT. Elohim (usually translated "God") and Adonai (in place of the sacred name Yawheh; and usually translated "LORD" in all caps in the OT at least with ESV Bible translation) are both used of God in the OT.Must Jesus be Lord,
or must He be GOD?
But the real 'smoking gun', however, is the substitution of Christ for God in OT passages.
For example, the Pre-Pauline "Christ Hymn" in Phillipians 2.6-11 believed to be an early Christian hymn that Paul is quoting:
Who, being in very nature[a] God,
did not consider equality with God something to be used to his own advantage;
7 rather, he made himself nothing
by taking the very nature[b] of a servant,
being made in human likeness.
8 And being found in appearance as a man,
he humbled himself
by becoming obedient to death—
even death on a cross!
9 Therefore God exalted him to the highest place
and gave him the name that is above every name,
10 that at the name of Jesus every knee should bow,
in heaven and on earth and under the earth,
11 and every tongue acknowledge that Jesus Christ is Lord,
to the glory of God the Father.
The hymn in turn quotes from Isaiah 45, which emphasizes monotheism and attributes such high praise to God and God alone.
5 I am the Lord, and there is no other;
apart from me there is no God.
23 By myself I have sworn,
my mouth has uttered in all integrity
a word that will not be revoked:
Before me every knee will bow;
by me every tongue will swear.
24 They will say of me, ‘In the Lord alone
are deliverance and strength.’”
So, in the Isaiah 45 passage where Yaweh alone is the one "every knee will bow to" and "every tongue confess," this early Christian hymn substitutes Jesus Christ, making it unmistakably clear that by Lord, the early Christians did, in fact, mean Jesus is God.
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