The Bible’s supreme proof text for telling the difference between the One God and the Messiah who is not God
This verse was referred to the Messiah by the Pharisees and by Jesus. It tells us that the relationship between God and Jesus is that of Deity and non-Deity. The Messiah is called adoni (my lord) and in every one of its 195 occurrences adoni (my lord) means a superior who is not God. Adonai on the other hand refers exclusively to the One God in all of its 449 occurrences. Adonai is the title of Deity and adoni never designates Deity.
If the Messiah were called Adonai this would introduce “two Gods” into the Bible and would be polytheism. Psalm 110:1 should guard us all against supposing that there are two who are God. In fact the Messiah is the supreme human being and agent of the One God. Psalm 110:1 is the Bible’s master text for defining the Son of God in relation to the One God, his Father.
Why is it that a number of commentaries misstate the facts about Psalm 110:1? They assert that the word for the Messiah in Psalm 110:1 is adonai. It is not. These commentaries seem to obscure a classic text defining God in relation to His Son. The Hebrew text assigns to the Messiah the title adoni which invariably distinguishes the one addressed from the Deity. The Messiah is the supreme human lord. He is not the Lord God (cp. I Tim. 2:5; I Cor. 8:4-6; Mark 12:28ff).
Why is the Messiah called adoni (my lord) and never adonai (my Lord God)?
“Adonai and Adoni are variations of Masoretic pointing to distinguish divine reference from human.”
Adonai is referred to God but Adoni to human superiors.
Adoni — ref. to men: my lord, my master [see Ps. 110:1]
Adonai — ref. to God…Lord (Brown, Driver, Briggs, Hebrew and English Lexicon of the Old Testament, under adon [= lord]).
“The form ADONI (‘my lord’), a royal title (I Sam. 29:8), is to be carefully distinguished from the divine title ADONAI (‘my Lord’) used of Yahweh.” “ADONAI— the special plural form [the divine title] distinguishes it from adonai [with short vowel] = my lords” (International Standard Bible Encyclopedia, “Lord,” p. 157).
“Lord in the OT is used to translate ADONAI when applied to the Divine Being. The [Hebrew] word…has a suffix [with special pointing] presumably for the sake of distinction…between divine and human appellative” (Hastings Dictionary of the Bible, “Lord,” Vol. 3, p. 137).
“Hebrew Adonai exclusively denotes the God of Israel. It is attested about 450 times in the OT…Adoni [is] addressed to human beings (Gen. 44:7, Num. 32:25, II Kings 2:19 [etc.]). We have to assume that the word adonai received its special form to distinguish it from the secular use of adon [i.e., adoni]. The reason why [God is addressed] as adonai, [with long vowel] instead of the normal adon, adoni or adonai [with short vowel] may have been to distinguish Yahweh from other gods and from human lords” (Dictionary of Deities and Demons in the Bible, p. 531).
“The lengthening of the ā on Adonai [the Lord God] may be traced to the concern of the Masoretes to mark the word as sacred by a small external sign” (Theological Dictionary of the OT, “Adon,” p. 63 and Theological Dictionary of the NT, III, 1060ff. n.109).
“The form ‘to my lord,’ l’adoni, is never used in the OT as a divine reference…the generally accepted fact that the masoretic pointing distinguishes divine references (adonai) from human references (adoni)” (Wigram, The Englishman’s Hebrew and Chaldee Concordance of the OT, p. 22) (Herbert Bateman, “Psalm 110:1 and the NT,” Bibliothecra Sacra, Oct.-Dec., 1992, p. 438).
Source: Adonai and Adoni (Psalm 110:1)