III. The cessation of active persecution does not do away with the need of continued vigilance: Satan has only changed his tactics
Such courage as this, dearly-beloved, we do not believe to have been needful only at those times in which the kings of the world and all the powers of the age were raging against God's people in an outburst of wickedness, thinking it to redound to their greatest glory if they removed the Christian name from the earth, but not knowing that God's Church grows through the frenzy of their cruelty, since in the tortures and deaths of the martyrs, those whose number was reckoned to be diminished were augmented through the force of example. In fine, so much strength has our Faith gained by the attacks of persecutors that royal princedoms have no greater ornament than that the lords of the world are members of Christ; and their boast is not so much that they were born in the purple as that they have been re-born in baptism.
But because the stress of former blasts has lulled, and with a cessation of fightings a measure of tranquillity has long seemed to smile upon us, those divergences are carefully to be guarded against which arise from the very reign of peace. For the adversary having been proved ineffective in open persecutions now exercises a hidden skill in doing cruel hurt, in order to overthrow by the stumbling-block of pleasure those whom he could not strike with the blow of affliction. And so seeing the faith of princes opposed to him and the indivisible Trinity of the one Godhead as devoutly worshipped in palaces as in churches, he grieves at the shedding of Christian blood being forbidden, and attacks the mode of life of those whose death he cannot compass.
The terror of confiscations he changes into the fire of avarice, and corrupts with covetousness those whose spirit he could not break by losses. For the malicious haughtiness which long use has ingrained into his very nature has not laid aside its hatred, but changed its character in order to subjugate the minds of the faithful by blandishments. He inflames those with covetous desires whom he cannot distress with tortures: he sows strifes, kindles passions, sets tongues a-wagging, and, lest more cautious hearts should draw back from his lawless wiles, facilitates opportunities for accomplishing crimes: because this is the only fruit of all his devices that he who is not worshipped with the sacrifice of cattle and goats, and the burning of incense, should be paid the homage of various wicked deeds.
Source: Sermons (New Advent)