"These signs will accompany those who have believed: in My name they will cast out demons, they will speak with new tongues; they will pick up serpents, and if they drink any deadly poison, it will not hurt them; they will lay hands on the sick, and they will recover."
My son had a snake once, but it was a boa, not a rattler. I picked it up many times and was not harmed.
I also studied Spanish in school, but I was never very good at it.
However, to really address the root of your question, what exactly is meant by "those who have believed"? Is it referring to the group as a whole? Or is it referring to each individual member of the group?
Example: Prior to the American Civil War, the Southerners had slaves.
Did each and every Southerner own multiple slaves? If each Southerner owned multiple slaves, then each slave, also a Southerner, owned multiple slaves. And each of those slaves owned multiple slaves.
Obviously, while the grammar could be taken to mean "each individual in the South", it clearly does not mean that. Logic prohibits that meaning, as demonstrated in the paragraph above. History tells us that the South, as a whole, permitted slave ownership, but that every Southern individual did not necessarily participate in the role of slave-owner. With a few exceptions, only wealthy white people owned slaves. According to one report, only 4.8% of Southerners owned slaves in 1860. Whether that figure is entirely accurate or not is not the point -- the point is that if it is an accurate figure, it would still not be incorrect to say "Prior to the American Civil War, the Southerners had slaves", even though only 4.8% of them did.
Likewise, "those who have believed" need not mean 100% of those who believed. It can mean only some of those who believed. But just because it grammatically COULD mean this, why should we accept that THIS IS what it means?
The Apostle Paul informs us that not every Christian speaks in tongues (1 Corinthians 12). Therefore, we know from the testimony of scripture that the words spoken by Christ in Mark 16 cannot possibly mean "every single person who believes". Rather, it means that amongst believers, these things will present themselves. And, indeed, many of these things occurred in the scriptural record, as the references below demonstrate.
In My name they will cast out demons: Mark 6:13; Acts 5:16, 8:7, 16:18, 19:12.
They will speak with new tongues: Acts 2:4, 10:46, 19:6; 1 Corinthians 14:18.
They will pick up serpents and it will not hurt them: Acts 28:3.
They will lay hands on the sick, and they will recover: Mark 6:13, Luke 9:6, Acts 3:7, 5:16, 8:7, 28:8.
Note that the disciples did not make a practice of seeking out poisonous snakes just to hold them and see if they could survive a snake bite. Nor did they make a practice of mixing up deadly poisons to drink to prove God's power. In fact, the Bible commands Christians not to behave in such a foolish manner when it says "you shall not put the Lord your God to the test" (Deuteronomy 6:16; Matthew 4:7; Luke 4:12).