Venerable Porphyrios of Kavsokalyva said the following about Old Calendarism:
"If you are going to live in the world, follow the calendar of the Church, that you may not bring turmoil to the people, unless you want to live on Mount Athos."*
Even if you live on Mount Athos, the venerable Elder encouraged the following: Be in communion with the Church and the Monasteries and do not live in the delusion of "zealotry", for it is because of your indiscriminate zealotry that there is a split within the Church, each person raising their own banner to indicate their egoism, and not an Orthodox mindset which is adorned with humility and obedience.
This is why he stressed:
"To preserve our unity, we must be obedient to the Church, to her Bishops. When we are obedient to the Church, we are obedient to Christ. We must suffer for the Church. Let us not accept the criticisms of her representatives.
At Mount Athos the spirit I learned was Orthodox, deep, holy, silent, without quarrels, without fights, without condemnation. Do not believe the critics of the clergy. Even if we see with our eyes something negative done by a cleric, let us not believe it, nor think about it, nor communicate it. The same goes for the lay members of the Church and for all people. We are all the Church. Whoever criticizes the Church because of the mistakes of her representatives, in order to supposedly help and correct her, make a big mistake. These people do not love the Church. And of course, Christ either. We love the Church when with our prayers we embrace each member and do whatever Christ does. We sacrifice ourselves, we keep vigil, we do everything like Him, Who 'when they hurled their insults at Him, He did not retaliate; when He suffered, He made no threats"' (1 Pet. 2:23).
* The Church of Greece, to which St. Porphyrios belonged while in Athens, follows the New Calendar, while most of Mount Athos chooses to follow the Old Calendar, as it is tolerated by the Ecumenical Patriarchate.
Source: From the book Spiritual Therapy According to Saint Porphyrios. Translation by John Sanidopoulos.