I would say it means everyone in a state of sin, even those who are devout Catholics who go to confession daily. The second you sin, you need to repent and “convert”.
The Catholic Church holds the fullness of Faith, true. But what happens between GOD and a fellow human is between them. Just because someone is/ is not Catholic does not mean they have a perfect relationship with GOD.
Sure, merely being Catholic doesn’t mean you have a perfect relationship with God- but surely a practicing Catholic has a more perfect relationship with God than a practicing insert-other-faith-tradition-here? It’s difficult to have a great relationship with someone you don’t fully understand.
Doesn’t God want what is best for us? And wouldn’t it be best for us to be in the Church He founded and continues to guide, rather than one which broke away? Anything else is imperfect.
Which protestant denomination are you refering to? I do not have the answer to that my friend. We are just going to have to pray to the Holy Spirit for unity.
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.