St. Ambrose ends the first book of his treatise On the Holy Spirit with the sixteenth chapter. In it he continues to play with the metaphor of God as a fount that pours forth Living Water (see John 4:14). St. Ambrose observes:
Good, then, is this water, even the grace of the Spirit. Who will give this Fount to my breast? Let it spring up in me, let that which gives eternal life flow upon me. Let that Fount overflow upon us, and not flow away. For Wisdom says: ‘Drink water out of thine own vessels, and from the founts of thine own wells, and let thy waters flow abroad in thy streets’ (Prov. 5:15, 16). How shall I keep this water that it flow not forth, that it glide not away? How shall I preserve my vessel, lest any crack of sin penetrating it, should let the water of eternal life exude? Teach us, Lord Jesus, teach us as Thou didst teach Thine apostles, saying: ‘Lay not up for yourselves treasures upon the earth, where rust and moth destroy, and where thieves break through and steal’ (Matt. 6:19).
It is here that we find the concluding argument of St. Ambrose against his theological opponents:
Our moth is Arius, our moth is Photinus, who rend the holy vesture of the Church with their impiety, and desiring to separate the indivisible unity of the divine power, gnaw the precious veil of faith with sacrilegious tooth.
In other words, we cannot have a proper relationship with God if we do not have a proper (dogmatic, Orthodox) understanding of who God is. Having an improper understanding of God as like having a broken cistern — unable to hold the living water that is God.
To use another analogy, imagine a gold fish and a golden retriever. Both are pets, both can be described using the word gold. As pets, we care for, feed and provide shelter for both; however, our relationship with each is radically different. If we were to try and have the kind of relationship we would expect to have with a dog with a fish (or vice-versa), the results would be disastrous. To apply this analogy to what St. Ambrose is saying, the likes of Arius and Photinus (and others who argue against a dogmatic and Orthodox understanding of God) would have us treat a fish like a dog.
Therefore, it is imperative that we defend the faith that has been handed down to us. It is critical that we maintain a dogmatic and Orthodox understanding of God. Note how St. Ambrose ends his first book of On the Holy Spirit:
If you seek Jesus, forsake the broken cisterns, for Christ was wont to sit not by a pool but by a well. There that Samaritan woman (John 4:6) found Him, she who believed, she who wished to draw water. Although you ought to have come in early morning, nevertheless if you come later, even at the sixth hour, you will find Jesus wearied with His journey. He is weary, but it is through thee, because He has long sought thee, thy unbelief has long wearied Him. Yet He is not offended if thou only comest, He asks to drink Who is about to give. But He drinks not the water of a stream flowing by, but thy salvation; He drinks thy good dispositions, He drinks the cup, that is, the Passion which atoned for thy sins, that thou drinking of His sacred blood mightest quench the thirst of this world.