Even in the 19th Century, Unitarians believed in the "Atonement." Again, it's not unlike with Mormonism and Jehovah's Witnessism; in a broad sense they believe in many of the same things as the "orthodox." But when specifically defining terms, they mean irreconcilably different things. The Unitarians did NOT believe Christ as 2nd Person in the Trinity made an infinite Atonement as necessary satisfaction for the infinite penalty cosmic justice demands because man sinned against an infinite God. Rather something else:
ATONEMENT AND RECONCILIATION.
§ 40. Unitarians believe that atonement and reconciliation are the same thing. Both mean a state of union and peace between man and God; the harmony between the Divine justice and Divine mercy; and the substitution of trust toward God and dependence on him, for fear and the dread of his displeasure.
§ 41. Unitarians do not believe that Christ came to reconcile God to man, but to reconcile man to God; not to make God love us, but to reveal his love; not to harmonize his justice and mercy, but to show that they are always in harmony. Christ's death was not a sacrifice made to appease the Divine anger, but it was an expression of the Divine love. Paul says (Rom. viii. 32), "He that spared not his own Son, but delivered him up for us all, how shall he not with him also freely give us all things?"
Hopefully one can now understand how Arians like Jonathan Mayhew, Samuel West and many others from the Founding era could disbelieve in Original Sin, Trinity, and Incarnation but hold to an unorthodox view of the Atonement. Likewise they believed in the Resurrection and even Christ's "divine" nature. Christ was "divine" but created and subordinate to the Father. Lower than God but higher than the highest arch-angel. (So quotations that refer to Christ as "divine" are NOT smoking gun proofs of orthodoxy; rather one must prove the speaker believed Christ God the Son, 2nd Person in the Trinity.)
Update: I probably should have included a Founding era as opposed to a late 19th Century era quotation on unitarians and the atonement. My reasoning was this: Unitarianism seemed to (?) become even more "liberal" as time passed. Therefore IF during the mid-late 19th Cen. they still believed in something they called atonement, it's no stretch to say that many unitarians in the 18th Cen. believed it. Indeed, Jonathan Mayhew, the militant Arian he, went on at great length explaining how he believed in what he understood as the "atonement." You can read him arguing with an orthodox figure who accuses him of denying the atonement. As I understand it, 1. Mayhew clearly says he believes in (and preached) the "atonement," 2. but when explaining just how he understands the doctrine, intimates an "unorthodox" understanding of the "atonement."
I chose the 19th Cen. quotation because it reads clearer than Mayhew's.