Just a quick note to help in countering the assertions made by Ghosty and others that the essence/energy distinction cannot be found in the early Church Fathers with any precision.
Here is a quote from Saint Basil the Great who is mid-4th century -and that should be early enough.
“We say that we know the greatness of God, His power, His wisdom, His goodness, His providence over us, and the justness of His judgment, but not His very essence… The energies are diversified, and the essence simple, but we say that we know our God from His energies, but do not undertake to approach near to His essence. His energies come down to us, but His essence remains beyond our reach… So knowledge of the divine essence involves perception of His incomprehensibility, and the object of our worship is not that of which we comprehend the essence, but of which we comprehend that the essence exists.”
Letter to Amphilochius