> The argument I’ve heard is that a Christian only answers to the authority of Christ but Catholics answer to the pope

That's a really nonsensical argument. Most non-Catholic churches have authorities – Bishops, Conventions, whatever – just because they don't accept the authority of the Pope doesn't mean they don't accept have other ecclesial authorities which they do accept.

Also, Eastern Orthodox theologians–and even some Protestant theologians–have said they accept the Pope's authority in principle, but they cannot accept it in practice because the Popes have abused their power and given themselves far more power than they were ever meant to have – but, in principle, if the Catholic Church turned over a new leaf, and adopted a much more limited view of papal power, and if their other theological disputes with Catholicism could be satisfactorily resolved – then they'd be able to go back to accepting the Pope's authority, not just in principle, but in practice too.