The dark ages?

From Spurgeon's sermon, "The Standard Uplifted
in the Face of the Foe" No. 718. Isaiah 59:19.

In 325, Emperor Constantine, for reasons of state
craft and subtle policy, made Christianity the national
religion, and thus struck the most fearful blow at
the vitals of Christianity. The union of church and
state is a fatal blow to true religion. The king's
hand wherever it falls upon the church of Christ
brings the king's evil with it. There never was a
church whose spirituality survived it yet, and there
never will be.

Christ's kingdom is not of this world, and if we
try to marry the church of Christ to a worldly
kingdom, we engender innumerable mischiefs.

So it happened that when the church became
outwardly glorious she became spiritually debased.
Her communion table glittered with gold and silver plate,
but her communion with Christ was not so golden as
before. Her ministers were enriched, but their doctrine
was impoverished. For every ounce of outward gold which
she gained, she lost a treasure of grace. Her bishops
became lords, and her flocks were famished. Her humble
meeting-places were exchanged for grand basilicas, but
the true glory was departed.

She became like the heathen around her, and began
to set up the images of her saints, until at last, after
years of gradual declension, the Church of Rome
ceased to be the church of Christ, and that which
was once nominally the church of Christ actually
became the Antichrist.

Black darkness covered the lands, and the dark ages set
in. Instead of pardon bought with the blood of Jesus, false
priests made merchandise of souls, and pardons were
hawked in the streets. Instead of deacons and elders
adorned with holiness and purity; monks, and nuns, and
priests, and even popes became monsters of filthiness.
Instead of justification by faith, men proclaimed justification
by pilgrimages and by penances. The crucifix took the place
of Christ Jesus, and a piece of bread was lifted up as a god,
and men bowed before it, and said, "These be your gods, O
Israel, that redeemed you from the wrath to come."