The Belgic Confession (1619) was written by a Belgian theologian named Guy de Bray, who was killed for his faith at the age of 45, in the year 1567. He was a student of John Calvin and Theodore Beza. This confession of faith is widely used by Reformed churches throughout the world.
We all believe with the heart, and confess with the mouth, that there is one only simple and spiritual Being, which we call God; and that he is eternal, incomprehensible, invisible, immutable, infinite, almighty, perfectly wise, just, good, and the overflowing fountain of all good.
Article 1, Belgic Confession
I am surprised he didn’t define God as triune. He discusses the Trinity later, of course (Article 8). But, I’m not sure why de Bray didn’t make sure he emphasized that God’s triunity is essential to who He is.