So the NWT itself is saying that Jehovah and Jesus have the same title "Mighty God". How can this be if there is only one God?
I am not with the JWs, but I do agree with them concerning the trinity, and that Jesus is not Jehovah.
How could Jehovah and Jesus have the same time of "Mighty God"? In a similar way that the rulers spoken of in Ezekiel 32:21 have the same title. The same Hebrew phrase is used also in Ezekiel 32:21, although gibbor is plural in this verse, (elei gibborim) to accommodate the fact that more than one is being spoken of. In this verse, as far as we know, no English translation renders the Hebrew word EL as "God", but rather with its general meaning of "might", "power", "strong", etc. The King James Version renders the Hebrew phrase (two Hebrew words) in Jeremiah by using five English words: "The strong among the mighty". In other words the King James translators recognize that the word EL here is not referring to the Supreme Being, but rather that it is being used of generic "strength", thus they rendered it as "strong". Similarly, the World English Bible translation renders the verse as:
The strong [ELIE] among the mighty [GIBBORIM] shall speak to him out of the midst of Sheol with those who help him: they are gone down, they lie still, even the uncircumcised, slain by the sword.
Jay Green's interlinear (word-for-word) rendering is: shall speak to him The strong [EL - Strong's #410] of the mighty [gibbowr - Strong's #1368] from the of midst Sheol.
Here we have the phrase El GIBBOR, although it is plural in the Hebrew, it is the basic same phrase of Isaiah 9:6, speaking pictorially of the kings who had already lost their rulership to the king of Babylon, as though they were in a death-like condition in sheol. The New Living Bible translates this phrase as "mighty leaders". The New Century translation renders it: "the leaders of the mighty ones." Rotherham renders it: "the chiefs of the mighty." The Geneva Bible renders it: "The most mighty & strong." The Bishop's Bible translates it: "The mightie worthies." Wycliffe renders it: "The myytieste of stronge men." Most translations render it something like "the strong among the mighty". Using these translations of Ezkiel 32:21 as a pattern, EL GIBBOR in Isaiah 9:6 could also be rendered "Mighty and Strong", or "Mighty One of Strength", especially since the context shows that the one being spoken of is not the only true God, Yahweh. However, with the reasoning often presented, that EL GIBBOR can only apply to Yahweh, then by this same manner of reasoning we should conclude that all the kings pictured as speaking from sheol are Yahweh the only true God. The fact is that the reasoning is false; EL, when used of others than Yahweh, takes on a general meaning of strength, might, power, etc., and thus EL GIBBOR can be used of others without designating them as the Supreme Being.
As I have pointed out on the RL website*, the word El has a dual meaning, one of which is the Might of the universe (Supreme Being), and the generic meaning of "strength", "power", "might", etc. This word is Hebrew #410 in Strong's Hebrew and Chaldee Dictionary, which defines it as: "short. from 352; strength; as adj. mighty; espec. the Almighty (but used also of any deity)." Crosswalk's Online Hebrew Lexicon defines it similarly: "shortened from 0352 TWOT - 93a n m ; AV - God 213, god 16, power 4, mighty 5, goodly 1, great 1, idols 1, Immanuel + 06005 2, might 1, strong 1; 245; 1) god, god-like one, mighty one; 1a) mighty men, men of rank, mighty heroes; 1b) angels; 1c) god, false god, (demons, imaginations); 1d) God, the one true God, Jehovah; 2) mighty things in nature; 3) strength, power". (http://bible.crosswalk.com/Lexicons/Hebrew/heb.cgi?number=0410) Other authorities give it the same or similar meanings. Consequently it is applicable to any powerful being and especially so the most powerful - the Almighty Yahweh.
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*See the study on "The Hebraic Usages of the Titles for 'God'":
http://godandson.reslight.net/hebraictitles.html In service of Yahweh and his Son, Ronald