Has a relationship always been in existence, even before time itself was created? Is a relationship the greatest of all truths?
“If nothing compelled God to create, why then did he choose to do so? In so far as such a question admits of an answer, our reply must be: God's motive in creation is his love. Rather than say that he created the universe out of nothing, we should say that he created it out of his own self, which is love. We should think, not of God the Manufacturer or God the Craftsman, but of God the Lover. Creation is an act not so much of his free will as of his free love. To love means to share, as the doctrine of the Trinity has so clearly shown us: God is not just one but one-in-three, because he is a communion of persons who share in love with one another. The circle of divine love, however, has not remained closed. God's love is, in the literal sense of the word, "ecstatic" a love that causes God to go out from himself and to create things other than himself. By voluntary choice God created the world in "ecstatic" love, so that there might be besides himself other beings to participate in the life and the love that are his.”
~ Bishop Kallistos Ware, The Orthodox Way
The following is the strongest, most concise, logical argument I can put forward for the One True God being in a relationship before he created anything.
God is complete in himself and does not need anyone or anything.
If a relationship has not always existed, God is not a relational being or he is not complete in himself.
If God is a relational being, but consists of only one person, then there was a time when God was not in a relationship, and therefore was not complete in himself. (But God is complete in himself; he does not need anything.)
If God is complete in himself, but is not a relational being, then there is no reason for him to create. He does not need to be entertained or worshipped. What could he possibly desire if he is not a relational being? He has nothing to prove, nor does he care for the praise of men.1 (If God is not a relational being—but decided to create just for the hell of it—then he is the ultimate terror, for we mean no more to him than inanimate objects.)
But if God is a relational being, and he is complete in himself, then he is love; and this love has always existed in a perfect relationship. And it is this love that created us, and it is to this love we belong.
“But you are only using logic to arrive at your conclusions. What does the Bible teach?”
Good question. The book Jesus: A Biblical Defense of his Deity by McDowell and Larson answers that question.
Note:
The Bible teaches that God is a relational being, and yet he is complete in himself. He does not need anything outside of himself; he is not dependant on us. Being a relational being, he desired that others could share in the relationship he has always enjoyed, and so he created. Because he is a relational being who loves us we can hurt him, but we cannot cause him to do wrong. God is the source of his own strength.
YHWH is not the third of three Gods. YHWH is one God; one God who consists of more than one person. This is beyond comprehension, and yet it is the only view which explains why God would create. (For more on this see here. See also Why Believe Jesus Christ is God).
It is also worth noting that there was a time when evil did not exist. And there will be a time when evil ceases to exist. Good can exist without evil, but evil cannot exist without good. (C.S. Lewis explains why in his book Mere Christianity.) Btw, when I say evil will cease to exist, I do not mean it will simply be quarantined. It will completely cease to be. See below.
1. God wants people to praise him for their good, not for his. If we worship anything that is less than God, we are seeking something less than what we were made for. See Maximus' Theory of Sin - Jordan Daniel Wood