Arthur Conan Doyle was the foremost defender of spiritualism during his lifetime. He saw séances as a way to provide both comfort and guidance: comfort to the survivors, as they could communicate with their departed loved ones; guidance to all humankind, as the spirits could provide information about the hereafter. The spirits invoked at the séance are like a minister in a church, providing help, comfort, and guidance to the attendees. The spirits are the angels who minister to the living. Hence, to Conan Doyle, spiritualism was the ministry of angels.
Conan Doyle defends spiritualistic practices against mainstream Christian belief. The Church itself had very mixed feelings about spiritualism, and many condemned it as contrary to Christianity. For example, a 1937 report of some Church officials, unpublished until the 1960s, said:
We cannot avoid the impression that a great deal of Spiritualism as organised has its centre in man rather than in God, and is, indeed, materialistic in character. To this extent it is a substitute for religion, and it not in itself religious at all. We were impressed by the unsatisfactory answers received from practising Spiritualists to such questions as, "Has your prayer life, your sense of God, been strengthened by your Spiritualistic experiences?" This explains in great part the hesitancy of many Christians to have anything to do with it.
According to traditional Christian belief, the dead rest until the day of Resurrection. Spiritualism could easily be seen as going against this because it involved waking the dead. Conan Doyle says that a newspaper piously called it exactly that, thereby encouraging hooligans to disrupt séances. The word pious has connotations of hypocrisy. Here are excerpts from the definition at Merriam-Webster:
: marked by conspicuous religiosity
// a hypocrite—a thing all pious words and uncharitable deeds
— Charles Reade
: marked by sham or hypocrisy
: marked by self-conscious virtue : VIRTUOUS
So Conan Doyle says that the newspaper hypocritically called spiritualism an unholy act whereas in reality it is the Ministry of Angels.