Grassi was later ordained to the priesthood on 17 December 1617 and received ordination from the Bishop of San Miniato Alessandro Strozzi.

He also spent hours in the confessional and claimed that the main task of a priest was to help and to console parishioners who needed to confess their sins.

The Governor admired him to the point of putting Grassi's portrait in the town hall as a form of commemoration and recognition for his services as a mediator between warring factions.

During this pilgrimage he visited the churches and other places that were related to the life of Saint Philip Neri; he learnt much more about him from Father Consolini who served as a guide.

Soon word of his personal holiness would reach Urban VIII and his advisors and this won Grassi the esteem of the pope.