We believe in one God, the Father, the Almighty, the Maker of Heaven and Earth, and of all things visible and invisible.
There is a lot we could say about this first proposition, and it will be useful to go to some decent sources and understand the context in which these particular words were framed.
But I immediately take from these positive propositions some negative constraints. We do believe positively in one God. But negatively we do not believe in many Gods. The proposition does not positively exhaust all that can be said about this one God, indeed there is a great deal more that can be said, but a vast area of theology is excluded. We do not believe in many Gods, so all polytheistic systems are excluded. However we reflect on this proposition we may not wander into a polytheistic understanding.
Now I also suggest that we all read this proposition slightly differently. Someone with strong environmentalist leanings might well find the idea of God as creator resonates with them. What does that mean to them? Someone concerned about political and social breakdown might find that the idea of God as Almighty strikes a chord.
We start in different places, even with a single propositional sentence. The proposition does not explain everything, rather it constrains what we may say as we reflect. We may say many things about God, but we may not say that God is not, in some sense, the creator of all things. We may not say that He is one God among many.
But the things we do say will reflect our own personal background and experience, so that the proposition is lived out in our own personal Orthodox life, not at all unrelated to all others living out the same propositions, but nevertheless not entirely liable to systematicisation. If I were to write an Orthodox Systematics it would be MY explanation, my choice of what to include and what to miss out, my prioritisation of subjects, and not yours, or someone elses. This is not relativisation at all, there are clear boundaries to reflection. But if human words cannot absolutely explain and express the ineffable beyond-beingness of God then all human words must be in some sense provisional.
An Baptist will approach becoming Orthodox differently to an Anglican, or a Roman Catholic, or a Pentecostal. Likewise a man different to a woman and a child. A European different to an Asian. Even the first words of the creed will mean something a little different because meaning is more than acceptance of words or argument but is a reflection on those words and is therefore personal.
I'd better stop now or I will keep saying the same thing I think. I am very interested in how others read this first proposition of the creed, what strikes them first and most, and how it is part of their Christian life and experience.