Mark 16:18

...lest in his ornate style they may drink poison, as out of a choice goblet, at your instance, though not after your own example, because they know not precisely what it is you have drunk yourself, and what you have left untasted, and because, from your high character, they suppose that whatever is drunk out of this fountain would be for their health. For what else are hearing, and reading, and copiously depositing things in the memory, than several processes of drinking? The Lord, however, foretold concerning His faithful followers, that even "if they should drink any deadly thing, it should not hurt them." And thus it happens that they who read with judgment, and bestow their approbation on whatever is commendable according to the rule of faith, and disapprove of things which ought to be reprobated, even if they commit to their memory statements which are declared to be worthy of disapproval, they receive no harm from the poisonous and depraved nature of the sentences.

The taking up of serpents, that is, the destruction of serpents, both sensible and intelligible, as it is said in another place: "I give you authority to tread on serpents and scorpions," evidently, intelligible ones (Luke 10:19). However, the expression "they will take up serpents" …

They who by their good exhortations take away malice from the hearts of others, take up serpents. And when they hear pestilent persuasions, yet are not drawn thereby to evil works, they drink indeed what is deadly, but it shall not hurt them. They who, as often as they see their neighbors grow weak in good works, run to help them with all their strength, and by the example of their own deeds strengthen the lives of those who waver in their own actions; what else do they do but lay hands on the sick that they may recover? Which miracles are so much the greater as they are spiritual; so much the greater as by them not bodies but souls are raised to life. These signs therefore, dearest brethren, you can do by God's help if you will. For by those outward signs, life cannot be obtained by those who work them. For those bodily miracles sometimes show holiness, but do not make it; but these spiritual ones, which are wrought in the mind, do not show but make the virtue of life. The evil can have those, but only the good can enjoy these. Whence the Truth says of certain ones: Many will say to me in that day: Lord, Lord, have we not prophesied in your name, and in your name cast out devils, and in your name done many mighty works? And then I will confess to them: I never knew you; depart from me, you workers of iniquity. Do not therefore, dearest brethren, love signs which may be had in common with the reprobate; but love these miracles of charity and piety which we have spoken of, which are so much the safer as they are hidden, and for which the recompense with the Lord is so much the greater as the glory among men is less.

(ubi sup.) Are we then without faith because we cannot do these signs? Nay, but these things were necessary in the beginning of the Church, for the faith of believers was to be nourished by miracles, that it might increase. Thus we also, when we plant groves, pour water upon them, until we see that they have grown strong in the earth; but when once they have firmly fixed their roots, we leave off irrigating them. These signs and miracles have other things which we ought to consider more minutely. For Holy Church does every day in spirit what then the Apostles did in body; for when her Priests by the grace of exorcism lay their hands on believers, and forbid the evil spirits to dwell in their minds, what do they, but cast out devils? And the faithful who have left earthly words, and whose tongues sound forth the Holy Mysteries, speak a new language; they who by their good warnings take away evil from the hearts of others, take up serpents; and when they are hearing words of pestilent persuasion, without being at all drawn aside to evil doing, they drink a deadly thing, but it will never hurt them; whenever they see their neighbours growing weak in good works, and by their good example strengthen their life, they lay their hands on the sick, that they may recover. And all these miracles are greater in proportion as they are spiritual, and by them souls and not bodies are raised.

That is, they shall scatter before them serpents, whether intellectual or sensible, as it is said, Ye shall tread upon serpents and scorpions, which is understood spiritually. But it may also mean sensible serpents, as when Paul received no hurt from the viper. There follows: And if they drink any deadly thing, it shall not hurt them. (Luke 10:19) We read of many such cases in history, for many persons have drank poison unhurt, by guarding themselves with the sign of Christ. It goes on: They shall lay hands on the sick, and they shall recorer.