Echad, or in Hebrew, "אֶחָד," is the word Moses used in Genesis 2:24,
עַל-כֵּן, יַעֲזָב-אִישׁ, אֶת-אָבִיו, וְאֶת-אִמּוֹ; וְדָבַק בְּאִשְׁתּוֹ, וְהָיוּ לְבָשָׂר אֶחָד
"Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh."
Do you see the word "echad" in bold and enlarged print in the Hebrew verse above? (Hebrew reads from right to left, so that is why it is at the tail end of the sentence)
YHWH says one man and one woman shall become ONE (echad) flesh. The man and the woman become one, not in number, but united as one in marriage. Rational minds fully accept and understand this unity.
In the very first verses of the entire Bible, YHWH drops us some hints about His own complex unity (text in parentheses is mine):
1 "In the beginning God (Elohim) created the heavens and the earth.
2 Now the earth was formless and empty, darkness covered the surface of the watery depths, and the Spirit of God (Elohim) was hovering over the surface of the waters."
26 Then God (Elohim) said, “Let Us make man in Our image, according to Our likeness." Genesis 1, HCSB
Now, the word "Elohim" is a plural noun, which can be translated as "gods."
Plural, not singular. It is not "the plural of majesty."
Neither is it God speaking to the angels, because God alone does the creating, and it is God's image in which we were created, not God's plus angels'.
The "im" is not a plural of majesty, as Muslims like to claim. Rather, it is plural just as the Book of Writings is called "Ketuviim" and the Prophets are called "Neviiim" since "navi" is the singular form of the word. Ashkenazi Jews are called Ashkenazim, and Sephardic Jews are called Sephardim.
So let's apply the same usage to our very first verse above,
"Shema Yisrael YHWH Elohim (plural) echad (one) YHWH." Deuteronomy 6:4
"Hear, Israel, YHWH our God(s) One YHWH."
And there we have the complex unity of the Godhead from Moses' pen.
In the New Testament, Yeshua ha Mashiach fully reveals what the Old Testament had partly concealed:
Our Father, Son and Holy Spirit are ONE YHWH.
Now, when John begins his Gospel, he echoes Moses' opening words:
1 "In the beginning was the Word, and the Word was with God, and the Word was God.
2 He was with God in the beginning.
3 All things were created through Him, and apart from Him not one thing was created that has been created." John 1, HCSB
This is not at all coincidental. John is making it crystal clear Who Jesus is in the very first verses of his Gospel. John is saying Jesus is the very same YHWH Who created the heavens and the earth
The context of the verse is that YHWH has come down from heaven and has been speaking face to face with Abraham about the problem of the sin-filled Sodom and Gomorrah
After 10 righteous people cannot be found in Sodom and Gomorrah, YHWH decides to destroy the two cities. Here is what Moses writes:
כד וַיהוָה, הִמְטִיר עַל-סְדֹם וְעַל-עֲמֹרָה--גָּפְרִית וָאֵשׁ: מֵאֵת יְהוָה, מִן-הַשָּׁמָיִם
24 Then the LORD caused to rain upon Sodom and upon Gomorrah brimstone and fire from the LORD out of heaven;
Transliterating from the Hebrew, with text in parentheses mine:
24 Then YHWH (Who is on earth with Abraham) caused to rain upon Sodom and upon Gomorrah brimstone and fire FROM YHWH out of heaven.
What we have in the above passage is YHWH on earth Who calls for judgment to fall from the YHWH in heaven, and yet, we all know there is only ONE YHWH.
What do we have every time Jesus prays in the New Testament?
We have YHWH on earth in the form of a man, calling to His Father, YHWH in heaven. Whenever Jesus prays, the conversation between Father and Son simply continues, displaying the total unity of the Spirit in the One God, and in His power and love.
There. It's as simple as that! :)
The Jewish Concept of the Complex Unity of God