Why? That's a good question. Some of the most significant questions you can ask start with that word. Why do I exist? Why am I here? Why should I do this or that? Why questions involve all of our human experience. They're also really good questions in the realm of theology. One of my favorite why questions in theology is this: why did God reveal himself? It is such a wonderful fact that God did indeed reveal himself both generally (in the works of creation) and specially (by the Word of his mouth in covenant). That is how God reveals himself... but why did he? For the sake of convenience, let's just limit the scope to special revelation. Why did he reveal himself by the word of his mouth in covenant?
There are a number of potential answers to the question. Some may say that God must have revealed himself, there was no other option. By virtue of his creating mankind he had to specially reveal himself — he was obligated to do so in the nature of the situation. Though, this answer fails for on account of the creator-creature distinction. That is, the creator and the creature are so distinct from one another with the creator being altogether greater than the creature. The mere existence of the creature does not in any way force the hand of the creator. As the Confession rightly states, "the distance between God and man is so great" (WCF 7.1).
Another potential answer is one that the confession gives. WCF 7.1 states that God's condescension in special revelation was so that the creature could "have fruition of [God] as their blessedness and reward". That is, it is for the sake of man that God reveals himself. This is altogether true and beautiful and there is not a single phrase I would change in WCF 7.1. Still, this is not the ultimate answer for why God reveals himself by the word of his mouth in covenant.
The ultimate answer is this: God reveals himself by the word of his mouth in covenant for his own glory. This answer is one that I arrived at from two separate angles. On the one hand, it comes from the general fact of the world that I have long been convinced about, namely, God does everything he does for his own glory. It was John Piper who was the first person to awaken me to this biblical reality and it certainly is attested to in the Westminster Standards (WCF 2.2, 3.3, 3.7, and 4.1 to name a few). But this direct connection between God's doing all things for his glory and his self-revelation was one that I saw explicitly in Heb 1:1-4.
Heb 1:1-4 is among my favorite sections in the entire New Testament. Verses 1-3a position God as having one grand scheme of revelation throughout all of redemptive history leading up to Christ — the full and final revelation of God. Verses 3b-4, however, turn to view Jesus Christ as the exalted and glorified Son of God. Seeing that, it dawned on me. God reveals himself for his own glory. Now, this does not in any way go against what the Confession states in 7.1, rather, what the Confession states is the means to this ultimate end — God reveals himself for his own glory.
Yet, here we must ask a related question: how does God's self-revelation bring him glory? God's self-revelation brings him glory in the sense that his manifold perfections are displayed by his revelation and esteemed by his elect. As Herman Bavinck put it in his Reformed Dogmatics, "God reveals himself for his own sake: to delight in the glorification of his own attributes." God takes delight when his own attributes are glorified, esteemed, made much of by those who perceive them through his special self-revelation. When we read "Anyone who does not love does not know God, because God is love." in 1 John 4:8, one of the most essential things we are to do is to esteem and make much of the fact that God is love and love others ourselves and so esteem his love through our loving others.
What an impact this would have if we came to our Bibles with this goal: to make much of God. He is great and his self-revelation declares as much. What a difference it would make if when we opened our Bibles we prayed to the Lord saying, "Lord, make your name hallowed in this heart and with these hands in light of what I read today".