Point 3. Union with God is through his essence. Also Filioque.

The second and third of the four points of RC-EO disagreement related to the teachings of EO St Gregory Palamas (1296-1359) are, in their EO version:

O2. There is a real, ontological distinction between divine essence and divine energies. God is his essence and his energies. The divine essence is the cause of the divine energies, which are uncreated. Each divine Person is the divine essence and the divine energies.

O3. Deification (theosis) is union with God through his energies (uncreated energy of the Holy Spirit = uncreated grace), and not through his essence.

Let's assume for a moment that O2 is true, and examine O3 in light of the last statement in Jesus' priestly prayer:

"I made known to them your name, and I will continue to make it known, that the love with which you have loved me may be in them, and I in them." (Jn 17:26)

Jesus is asking that "the love with which the Father has loved Him" may be in us. Is Jesus talking about the love with which the Father eternally loves the Son in the Godhead, or about the love with which the Father loves the Son in his human nature? The answer is just two verses before:

"that they may see my glory that you gave me, because you loved me before the foundation of the world." (Jn 17:24)

and in case there were any remaining doubts related to which one is the nature whose glory Jesus is talking about (which should be clear from the past tense anyway), there is this also this verse:

"And now, Father, glorify me in your own presence with the glory that I had with you before the world existed." (Jn 17:5)

Therefore it is clear that "the love with which the Father has loved Jesus", which Jesus asks that "may be in us", is the love with which the Father eternally loves the Son in the Godhead. Now, assuming that there is a real, ontological distinction between divine essence and energies, the love with which the Father eternally loves the Son "ad-intra" must necessarily be at the level of the essence. And if this love is in us, then the proposition that we only interact with God's energies is false.

Now, let's try to learn more about that love by looking again at Jn 17:26. Jesus is placing that love on an equal standing with Himself regarding the desired presence of both in the disciples. This can be seen even more clearly in some translations of that verse, such as in the New Jerusalem Bible:

"so that the love with which you loved me may be in them, and so that I may be in them." (Jn 17:26b)

The only possible interpretation then is that the love with which the Father eternally loves the Son in the Godhead is the Holy Spirit. So Jesus is asking that the Holy Spirit may be in us. He is thus fulfilling at this moment the promise He had made earlier to the disciples:

And I will ask the Father, and he will give you another Advocate ("Paraclete") to be with you always, the Spirit of truth, whom the world cannot accept, because it neither sees nor knows him. But you know him, because he remains with you, and will be in you. (Jn 14:16-17)

And at this point, with the help of God and asking for the light of the Holy Spirit, we can address the issue of Filioque. Can the love with which the Father eternally loves the Son remain unidireccional? Will not the Son reciprocate by eternally loving the Father with the same love? Or does this truth revealed by Jesus apply only to the activity of the Son "ad-extra":

To this Jesus replied: "In all truth I tell you, by himself the Son can do nothing; he can do only what he sees the Father doing: and whatever the Father does the Son does too." (Jn 5:19)

If God is love, and if the Son is "the exact imprint of God's being" (Hb 1:3), there can be no possible doubt that the love with which the Father eternally loves the Son is eternally and equally reciprocated. And since this love is the Holy Spirit, this is a most clear and firm basis for affirming "that the holy Spirit is eternally from the Father and the Son, ... and proceeds from both eternally as from one principle and a single spiration." (Ecumenical Council of Florence, session 6)

So, the Filioque is central to the RC conception of God.