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MARKS OF THE CHURCH. The four essential notes that characterize the Church of Christ, first fully enumerated in the Nicene-Constantinople Creed; one, holy, Catholic, and apostolic. Since the Eastern Schism and the Protestant Reformation they have become means of identifying the true Church among the rival claimants in Christianity. Some writers add other notes besides the traditional four, e.g., St. Robert Bellarmine with a total of fifteen, including the mark of persecution.
 
The Fifteen (15) Marks of the Church
Developed by St. Robert Bellarmine (1542-1621)

1. Catholicity
The Church's Name, Catholic, Universal, and world wide, and not confined to any particular nation or people.
 
2. Antiquity
In tracing her ancestry directly to Jesus Christ.
 
3. Constant Duration
In lasting substantially unchanged for so many centuries.
 
4. Extensiveness
In the number of her loyal members.
 
5. Episcopal Succession
Of her Bishops from the first Apostles at the Last Supper to the present hierarchy.
 
6. Doctrinal Agreement
Of her doctrine with the teaching of the ancient Church.
 
7. Unity
Of her members among themselves, and with their visible head, the Roman Pontiff.
 
8. Holiness
Of doctrine in reflecting the holiness of God.
 
9. Efficacy of Doctrine
In its power to sanctify believers, and inspire them to great moral achievement.
 
10. Holiness of Life
Of the Church's representative writers and defenders.
 
11. Miracles
Worked in the Church and under the Church's auspices.
 
12. The gift of Prophecy
Found among the Church's saints and spokesmen.
 
13. Opposition Aroused On the Same Ground As Christ Was Opposed
 
14. The Unhappy End of the Church's Enemies
 
15. The Temporal Peace and Earthly Happiness

Of those who live by the Church's teaching and defend her interests.
 
The Traditional four marks of the Church are: One, Holy, Catholic, and Apostolic. St. Robert Bellarmine builds upon these marks, which establish whether or not a Church can be considered true or false.

I like these a lot. :)

In a word: Christendom!

Amen!

The Avignon Papacy was the period from 1309 to 1376 during which seven successive popes resided in Avignon.   
 
Would not the Avignon Papacy be considered a change in the Constant Duration of the Church, which expresses In lasting substantially unchanged for so many centuries?

No. The physical location of the Pope's residence is an accidental/nonessential quality of the Church. Furthermore, the change was not lasting as it ended with Pope Gregory XI who returned the See back to Rome. It was a brief disruption caused by political pressure.