I usually cite the letter from Adams to Thomas Jefferson, September 14, 1813 for Adams' quote saying even if he were on Mt. Sinai with Moses and God revealed the doctrine of the Trinity to him there, Adams still wouldn't believe it because 1+1+1 = 3 not 1. However, later in the letter Adams also says some choice things regarding salvation and how he rejects the idea that only those who accept Christ, God the Son as Savior and His Atonement are saved. Adams reasons it's absurd to conclude God would create man knowing 9/10 would be subject to eternal misery.
As he wrote:
God has infinite wisdom, goodness, and power; he created the universe; his duration is eternal, a parte ante and a parte post. His presence is as extensive as space. What is space? An infinite spherical vacuum. He created this speck of dirt and the human species for his glory; and with the deliberate design of making nine tenths of our species miserable for ever for his glory. This is the doctrine of Christian theologians, in general, ten to one. Now, my friend, can prophecies or miracles convince you or me that infinite benevolence, wisdom, and power, created, and preserves for a time, innumerable millions, to make them miserable for ever, for his own glory? Wretch! What is his glory? Is he ambitious? Does he want promotion? Is he vain, tickled with adulation, exulting and triumphing in his power and the sweetness of his vengeance? Pardon me, my Maker, for these awful questions. My answer to them is always ready. I believe no such things. My adoration of the author of the universe is too profound and too sincere. The love of God and his creation — delight, joy, triumph, exultation in my own existence — though but an atom, a molecule organ- ique in the universe — are my religion.
Howl, snarl, bite, ye Calvinistic, ye Athanasian divines, if you will; ye will say I am no Christian; I say ye are no Christians, and there the account is balanced. Yet I believe all the honest men among you are Christians, in my sense of the word.