by the power of this sacrament and the example of St. Felix
help us always to live a hidden life with Christ,
who lives and reigns with you, for ever and ever.
From what I can tell, this prayer seems to be new in the 1974 Roman-Franciscan Sacramentary; I find no version of it in the postcommunion or any other of the prayers for St. Felix in the 1954 or 1962 editions of the Missale Romano-Seraphicum.
The prayer strikes me because it was precisely in the public life that St. Felix came to be known for his sanctity, praying and working in the streets of Rome. He didn't live anything like what we usually think of as the 'hidden' sort of religious life. The prayer reminds us, however, that the life of Christian devotion, like all human intimacies, is always secret. The grace and goodness we see in each other are not our Christianity at its secret core, but the fruits of a Spousal life which is difficult or perhaps impossible to show to a third party. There can be no pornography of the spiritual life; it is an intimacy impossible to display to someone else. Indeed, both through my own personal experience and my work as confessor and spiritual director, I realize more and more that many times much of God's intimacy with us is hidden even from ourselves, and often this is good for us and an expression of his mercy.
Pray for us, St. Felix, that we may descend into our created hearts where our lives are now hidden with Christ in God.