He writes:

I read your article on Unam Sanctam. Being an evangelical considering whether to become a Catholic, I found it helpful, though I am still struggling to wrap my mind around it. I had a further question. unam Sanctam also says: "Therefore, if the Greeks or others should say that they are not confided to Peter and to his successors, they must confess not being the sheep of Christ, since Our Lord says in John 'there is one sheepfold and one shepherd." What does this mean, then and now, for non-catholic christians?

I think it means, basically, that insofar as you reject Peter and his successors, you are rejecting Christ.  However, the Greeks don’t—and can’t—wholly reject Peter and his successors since they accept the creeds, formulated by councils in union with Peter’s successors.

I would suggest a reading of the Decree on Ecumenism to get the best present formulation of the Church’s position with respect to the eastern Churches not in full communion with the Catholic Church:

In addition, I urge a reading of Ut Unum Sint by Pope John Paul II, which is all about the role of the Pope and is written, in particular, with a view to the Pope’s relationship with the Eastern Churches.