You can be a witness to Christ every day. You were tempted by the spirit of impurity but... you considered that chastity of spirit and body should not be soiled: you are a martyr or, in other words, a witness to Christ... You were tempted by the spirit of pride but, seeing the poor and needy, you were seized by tender compassion and preferred humility to arrogance: you are a witness to Christ. Better still: you have not given your witness in word alone but in deed as well.
What is the surest kind of witness? “Anyone who acknowledges that Jesus Christ came among us in the flesh” (cf. 1Jn 4,2) and who keeps the commands of the Gospel... How many there are each day of these hidden martyrs of Christ who confess the Lord Jesus! The apostle Paul knew that kind of martyrdom and witness of faith rendered to Christ, he who said: “Our boast is this, the testimony of our conscience” (2Cor 1,12). For how many people have made a confession of faith exteriorly but denied it interiorly!... So be faithful and courageous in interior persecutions so that you may also win the victory in exterior persecutions. There are “kings and rulers”, judges of formidable power, in the persecutions within, likewise. You have an example of these in the temptations undergone by our Lord (Mt 4,1ff.)
Sermon 20 on Psalm 118 ; CSEL 62, 467f.