The context is that He was speaking to the apostles. Did He mean that no Christian, ever, would suffer from snake-bites or poison, and that every Christian in all times would cast out devils, speak with new tongues, cause the sick to recover? I believe that these signs were granted to the earliest Christians as confirmations of the gospel they preached, at a time when the Bible was as yet incomplete. As John Gill wrote in his commentary on verse 17:
"And these signs shall follow them that believe,.... Not all of them, but some; and not always, only for a time; and which were necessary for the confirmation of the Gospel, and the establishment of Christianity in the world; and not only believing hearers, but believing ministers of the word, are chiefly designed; and this is said, for the encouragement both of those that preach the Gospel, and of them that hear, believe and obey. The Persic version, contrary to all others, reads, "ye shall show signs and wonders to them that believe not";".