ANSWER: The modern version of snake handling or serpent handling (the KJV Bible uses the word 'serpent') began as a religious ritual in the Appalachia region of the U.S. by George Hensley. He introduced the practice, based on a literal interpretation of Mark 16:17 - 18, to a Holiness movement church around 1910. Hensley became a credentialed minister of the church in 1915. After seven years of service, he left the denomination that credentialed him to form the first Pentecostal church to require ALL its members handle serpents as proof of their conversion.
Mark 16:17 - 18 (and to a much lesser extent Luke 10:19) are the only places in the Bible used to justify poisonous snake handling. Jesus spoke the two verses in Mark as part of his final instructions to his disciples (known as the 'Great Commission') before his ascension (16:15 - 18). After giving them the authority to preach the gospel and baptize (verses 15 - 16) he states in verse 17 that "these signs shall follow those who believe."
The signs that were to follow believers include the ability to cast out demonic spirits and the ability to speak in languages other than their own. They were promised that snakes and poisons could not harm them and that they had the ability to heal the sick.
Those who practice this religious ritual believe Mark 16:18 should be interpreted literally (taking the English words at face value) for its meaning. They view the verse as a promise from Jesus that serpents will not harm them (especially during the worship of God) because they are Christians. One of the main problems with this Biblical understanding is that the tragic history of using poisonous reptiles as part of the worship of almighty God CONTRADICTS the validity of their interpretation.
Many believers have DIED while handling snakes in a church meeting or service. George Hensley, mentioned previously, died from a poisonous snakebite in 1955. In 1998, an 'evangelist' who used serpents during services died from a timber rattlesnake bite. His wife had died from a bite three years earlier. In 2012, a pastor in the Pentecostal movement passed away after receiving a bite from a rattlesnake while leading an outdoor service.
In early 2014, a man named Jamie Coots, made famous through a popular cable TV show called 'Snake Salvation,' DIED from a poisonous bite. A Discovery Channel News article about his death stated those who have passed away from bites received while worshipping God likely number in the hundreds.
A correct understanding of a Bible passage requires studying ALL related pieces in God's word (Isaiah 28:10). The apostle Paul admonished his friend and fellow-evangelist Timothy to work at "rightly dividing" the Scriptures to discover their meaning (2Timothy 2:15). Any conclusions drawn from a particular verse must be in harmony with what the REST of God's word teaches.
Taking Mark 16:18 as a guarantee snake handling will not harm Christians, regardless of the circumstances, goes against the teaching found in the Bible that we are not to tempt God (Deuteronomy 6:16, Matthew 4:7, Luke 4:12, 1Corithinas 10:9). It is one thing to accidently encounter a poisonous reptile and be saved from death (as Paul was, see Acts 28:3 - 6) and something entirely different to knowingly put ourselves in harm's way such that it tests God's love.