Did Jesus give us the authority to trample on the enemy?

...Does Mark 16:17, 18 show that ability to heal the sick would be a sign identifying believers?

Mark 16:17, 18, KJ: “These signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues; they shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover.”

These verses appear in certain Bible manuscripts and versions of the fifth and sixth centuries C.E. But they do not appear in the older Greek manuscripts, the Sinaiticus and Vatican MS. 1209 of the fourth century. Dr. B. F. Westcott, an authority on Bible manuscripts, said that “the verses . . . are no part of the original narrative but an appendage.” (An Introduction to the Study of the Gospels, London, 1881, p. 338) Bible translator Jerome, in the fifth century, said that “almost all the Greek codices [are] without this passage.” (The Last Twelve Verses of the Gospel According to S. Mark, London, 1871, J. W. Burgon, p. 53) The New Catholic Encyclopedia (1967) says: “Its vocabulary and style differ so radically from the rest of the Gospel that it hardly seems possible Mark himself composed it [that is, verses 9-20].” (Vol. IX, p. 240)

There is no record that early Christians either drank poison or handled serpents to prove they were believers.

Why were such gifts as the ability to do miraculous healing given to first-century Christians?

Heb. 2:3, 4: “How shall we escape if we have neglected a salvation of such greatness in that it began to be spoken through our Lord and was verified for us by those who heard him, while God joined in bearing witness with signs as well as portents and various powerful works and with distributions of holy spirit according to his will?”

(Here was convincing evidence, indeed, that the Christian congregation, which was then new, was truly of God. But once that was fully established, would it be necessary to prove it again and again?)

1 Cor. 12:29, 30; 13:8, 13: “Not all are prophets, are they? . . . Not all have gifts of healings, do they? Not all speak in tongues, do they? . . .
Love never fails.
But whether there are gifts of prophesying, they will be done away with; whether there are tongues, they will cease . . . Now, however, there remain faith, hope, love, these three; but the greatest of these is love.”

(When they had accomplished their purpose, those miraculous gifts would cease.
But priceless qualities that are the fruitage of God’s spirit would still be manifest in the lives of true Christians.)....
[Reasoning, )w.org ]

It must be true since a Christian said so.