It’s the four characteristics, inseparably linked with each other, indicate essential features of the Church and her mission.
The issue, though, isn’t whether a particular group
claims that they have all four marks, since many groups who have broken away from the Church – and thereby formed their own denomination
without apostolicity – but rather, whether they actually
do have all four marks.
I believe @1ke stated it was what was necessary for a valid confection for Christ to become sacramentally present in the Eucharist.
It is necessary to have a priest who was validly ordained. Only those ordained by a bishop who himself was ordained in a Church that maintains an unbroken chain of apostolic succession can hope to claim to be “validly ordained.”
So, among the requirements, the first is “validly ordained priest.”
Someone said Lutheran’s might believe what they believe but that their beliefs were not valid… how can anyone know that but God?
In this case, we’re talking about valid priesthood. According to the definition of the Catholic Church, Lutherans don’t have valid priests and therefore, cannot possibly have a valid Eucharist. They might disagree, and have their own definition, and we’d look at that and respond, “OK, that’s
your opinion, but…”.
This seems like a pretty cut-and-dried thing, that doesn’t require access to a person’s
heart. Rather, it only requires access to the record of Luther’s assertions and actions.
Again, the question isn’t whether Lutherans
believe they have a valid Eucharist, it’s whether it meets the definitions set up by the Church they left, whose doctrines they followed until they said “nope; we’re outta here.”
Only God knows the real validity of a person’s faith.
Well… I’m not sure I agree. If I said that I believe that the Easter Bunny died on the cross to save you from your sins, would you need God to assess the validity of that assertion?
On the other hand, if the Church proceeded from Christ, and from His grant of authority to Peter and the Apostles, and they handed that authority down to their successors, and through that authority, they set in place doctrine and discipline, wouldn’t their teaching be authoritative? And, wouldn’t those who taught
different stuff therefore, by definition, be teaching untruth? Objectively, and without recourse to “things only God knows”?