Many atheists claim they will become Christians if there was a verifiable miracle that could be attributed to God. Of course, they also raise their standard of "verifiable" to the point where God could literally re-arrange the stars, and they'd blame aliens first (isn't that right, Richard Dawkins?), but that's beside the point. Is there a single verifiable miracle that would vindicate Christianity in the minds of any reasonable person?
As it turns out, the central miracle of Christianity is one of the easiest to verify. First, with the exception of a few conspiracy theorists, basically all historians acknowledge that Jesus was crucified. There's good reason for this. Aside from the Bible being eyewitness testimony of this, there are numerous other records from around the right time detailing His death, including from people who deny His claims to divinity, and claims that He rose again.
The Bible is both the most authoritative and reliable source on the historical Jesus, and it actually contains some very interesting information that allows us to verify His death. For example, not only do we know that the centurion pierced His side with a spear to verify His death, but we are told "at once, there came out blood and water" (John 19:34). With our modern medical knowledge, this tells us a lot. Specifically, this, and various other details, tells us that Jesus suffered hypovolemic shock, causing pericardial effusion (the gathering of fluid around the heart), and pleural effusion (the gathering of fluid around the lungs). In other words, the Gospels actually include medical details far beyond the capabilities of even a doctor in the first century. These details not only strongly support that the crucifixion happened, and that Jesus was genuinely dead, but that the Bible accurately records it.
So, Jesus was genuinely crucified and killed by the most efficient killers of His era. The next question is, did He rise again? The evidence strongly suggests He did. Many witnesses, both friendly and hostile, affirmed that they had witnessed Him alive and well, even under threat of torture, and martyrdom. They also appealed to other witnesses who, at the time, were still living. Paul, for example, wrote that many members of a large crowd who saw Jesus were still alive in his time (1 Corinthians 15:6).
This article is by no means extensive. If anything, I want to encourage you to look at the evidence for the resurrection yourself. Many people far more qualified than myself present the evidence in a far better way. Many of these people were, themselves, once vocal atheists, who upon doing the research, discovered that Christ really is risen, and therefore confess Him as Lord, receiving the gift of His grace. It is freely available to you, as to us who have already accepted it.