And that you may understand it to be said as a mystery and not in reference to the bare number that two are better than one, he adds a mystical saying, A threefold cord is not quickly broken*. For that which is threefold and uncompounded cannot be broken. Thus the Trinity, being of an uncompounded nature, cannot be dissolved; for God is, whatever He is, one and simple and uncompounded; and what He is that He continues to be, and is not brought into subjection.
Ambrose of Milan, The Letters of S. Ambrose, Bishop of Milan (trans. H. Walford; A Library of Fathers of the Holy Catholic Church; London; Oxford; Cambridge: Oxford; James Parker and Co.; Rivingtons, 1881), 464.