What is the Elohim of the Old Testament?

The Elohim of the Old Testament is the invisible Godhead—the Trinity in Spirit form. The Son is the image of the invisible God, and out of that invisible Trinity one person of the Godhead took on flesh and became visible, namely the Son.

Les explains that the Elohim of the Old Testament was that invisible Godhead, the Trinity in its spirit form. God is invisible, but out of that invisible Trinity stepped one person of the Godhead, who took on flesh and became visible. This is what Paul is talking about: the Son, the Lord Jesus as we know Him, is the image or visible appearance of the invisible God. He is the firstborn of every creature, before anything else ever was. By Him all things were created in heaven and earth, visible and invisible, whether thrones, dominions, principalities, or powers—everything was created by Him and for Him. Les warns that bringing this person of the Godhead down to the level of mankind's other gods is blasphemy, because He is the Creator, and one day every human being will know Him as such. The lost will meet Him at the great white throne and will not argue.

Colossians 1:15

Who is the image of the invisible God, the firstborn of every creature:

Is the Trinity/Elohim the invisible Godhead?

Yes. The Trinity, identified as the Elohim of the Old Testament, was the invisible Godhead, its spirit. Out of that invisible Trinity one person of the Godhead took on flesh and became visible, namely God the Son, the Lord Jesus, who is the image of the invisible God.