Here are some initial thoughts on criteria for defining "catholicity" for the purposes of the "Lutheran Catholicity" blog your comment:
1. The Scripture Principal. To be catholic a statement or practise must be scriptural - "we believe, teach and confess that the sole rule and standard according to which all dogmas together with all teachers should be estimated and judged are the prophetic and apostolic scriptures of the Old and New Testaments alone, as it is written in Psalm 119:105 "Thy Word is a lamp to my feet and a light to my path". And St Paul says in Galatians 1:8, "Even if an angel from heaven should preach to you a gospel contrary to that which we preached to you, let him be accursed." [Formula of Concord, Epitome, 1]
This does not mean that an explicit scriptural warrant is required for every statement or practise to be deemed catholic, since some aspects of doctrine are drawn by logical and necessary inference from scripture (e.g. infant baptism), while some practises in the church which do not contradict the scripture principal or the Gospel (see next) may be permitted due to the freedom that the Gospel grants to the people of God under the New Testament (e.g. vestments).
2. The Gospel Principal. To be catholic a statement or practise must be consonant with the Gospel, defined in the narrow sense, i.e. that we are freely justified before God for Christ's sake when we believe that we are recieved into God's favour and our sins forgiven on account of Christ, who by his death made satisfaction for our sins. This faith God imputes to us as righteousness (Romans chs. 3 & 4) [see Augsburg Confession, Art IV). Any statement or practise that contradicts God's Gospel can self-evidently not be catholic.
This does not mean that every statement or practise deemed to be catholic must give the most perfect expression possible to the Gospel, only that it is clearly consonant with the Gospel as defined above. It is acknowledged that under God's mysterious providence, and for his own good purposes, the fortunes of the Gospel have waxed and waned in the history of the church and the world, and therefore the church's understanding and reception of the Gospel has at times been darkened, though never completely extinguished. Even in such dark times, people were saved by clinging to God's Gospel, to his glory.
In particular, it is acknowledged that the Lutheran Reformation brought forth a deeper and clearer appreciation for and understanding of the Gospel and its application to the Christian life than had hitherto existed (except among the Apostles), which finds expression in the Lutheran confessional writings. However, it should not be expected that God's people who lived and wrote before the Reformation will use exactly the same terminology as the Reformation Fathers (see next). Also, many expressions of genuine catholicity can be found emanating from outside of the Lutheran communion since the Reformation, and such are welcomed as true evidences of catholicity.
3. The Historical Principal. Under God's providence, the church's understanding and reception of scripture, the Gospel and the articles of faith, has unfolded and deepened over time, often as different crises from within and without have engaged teh church's attention, bringing forth deeper insights into Scripture and the Gospel. Nevertheless, the Godly doctrine and practise in the life of the church which we call "catholicity" has run through the church's history like a golden thread through a tapestry, always present, though with differing degrees of brilliance in different times and places. It is this thread which we term catholicity and which we seek to trace.
This means that statements and practises deemed to be catholic must be evaluated with due reference to the historical position and prevailing culture of the writer/church at the time, although the Scripture and Gospel principles must prevail.
This also means that the church will not lightly discard aspects of its life sanctified, as it were, by long usage through its history, provided such historical practises do not contradict Scripture or the Gospel, although the church retains the right to revise such aspects of its life as time and place necessitate, that the Gospel may have free course to impact people of all times, places and cultures.
OK. While you ponder that, here's a visual expression of catholicity, if you will, the altar painting from the Torslunde church in Copenhagen, Denmark, which depicts several aspects of the Divine Service according to Lutheran practise. I was going to use this behind the title of the Lutheran Catholicity blog but it proved too colourful for the title to come through clearly. I may still use it somewhere else on the blog. Many years ago I used this in a seminary class to show how the Lutheran Reformation retained the use of vestments - it was a hot issue at the time, perhaps not so much anymore.
Now, any high church types out there, tell me what is "wrong" with the clergy in the picture (apart from the fact that the preacher does not appear to be vested; and I don't mean the sleeveless surplices either).