There's a t-shirt with a slogan which says, "Jesus is coming... Look busy!"

It's satirical of course, making a gentle mockery of the idea that if Jesus really was coming back today, you'd want it to appear like you were 'up to something good.'

Of course, it's also nonsense. If Jesus is who Christians claim He is, then He knows what we're up to all the time. But the idea that there is a point coming very soon when Jesus will reappear, is a pervasive one in evangelical culture.

Of course all of us orthodox Christians believe Jesus is going to return at some point in the future.

A large sub-set of Christians go much further, though. Rather than just thinking that at some point in the near future, Jesus will return, these Christians are obsessed with the so-called 'end times.'

This obsession seems to have reached fever pitch in the last 20 or 30 years. The idea has never been in the mainstream of Christian theology – it's a thoroughly modern invention – and one which isn't supported by orthodox Christian belief.

There are two manifestations of this obsession with the end times. The more extreme end of it has 'prophets' making predictions about the exact date that the world will end or the date that Jesus will return. We've seen a number of examples in recent years.

Five years ago, Harold Camping hit the international headlines when he had two attempts at predicting the end of the world. Neither of them came true of course, and Camping seemed to belatedly realise the folly of his pronouncements. "We humbly acknowledge we were wrong about the timing," said Camping.

The other manifestation of the end times obsession doesn't make predictions for an exact date that the world will end, but it does suggest that the end is nigh...

Exponents of this brand of eschatology (the study of the end of the age) regularly claim that certain geopolitical events, cosmic events or even changes in the weather indicate that Jesus is about to return.

The latest of these shrill warnings of the impending end times landed in my inbox this morning. It was from Dr Michael Youssef, the pastor, broadcaster and author, originally from Egypt and now based in Atlanta, Georgia.

A press release about his new book read, "Today, the world is moving away from freedom as the solution and toward authoritarian control, one-world government, and the concentration of power in a single authority—even a single human leader."

It goes on, "Dr Youssef's End Times book also unpacks and demystifies the book of Revelation, highlighting its relevance to our lives today and shedding light on how global events are playing into God's plans for the end times..."

Youssef is quoted as saying, "The Bible has predicted with accuracy the chaos we are seeing today... My prayer is that God will use this book to fill the reader with hope, faith, and an eager expectation of the Lord's return."

This may all sound harmless enough. However, it's part of a wider picture of end times prophecies and predictions which fuel paranoia and extremist political positions, such as bombing Iran, supporting Israeli extremists and advocating the destruction of Russia.

Youssef is the latest in a long line of figures proclaiming the 'signs of the times.' Three years ago, Texas megachurch pastor John Hagee received huge attention (and sold a lot of books) on the back of his claims that the appearance of four 'Blood Moons' on the dates of Jewish festivals meant something of vast significance was about to happen. "The coming four blood moons points to a world-shaking event that will happen between April 2014 and October 2015," claimed Hagee.

Attempts to retrofit the Iran nuclear deal onto this prediction looked pretty desperate and the rest of the world was left to conclude that Hagee was wrong – the supposed event hadn't happened.

However, Hagee and Youssef are small fry when it comes to the biggest end times show in town: the Left Behind series. Since the 1990s a series of fiction books and films have expounded the theory that the end of time is imminent.

Selling hundreds of millions of copies, Jerry B Jenkins and Tim LaHaye wrote 16 books, produced four films and even had video games and other spin-offs. The stories produced vast interest in the idea that Jesus' return is close.

LaHaye said, "Oh, it's true. I'm expecting the Lord. In fact, my wife and I have enjoyed almost 60 years of wedded bliss. We've had a wonderful marriage because we were both Christians and committed to God to begin with, and we'd like to see it perpetuated in the Millennium without interruption, be taken up in the Rapture."

This last point is a key to understanding why so many Christians now seem obsessed with the end times. So-called 'rapture theology' was developed in the 19^th Century by a British clergyman called John Nelson Darby. Darby's suggestion, based on a passage in 1 Thessalonians (which actually means the opposite of what Darby claimed it meant), was that Christians would be 'raptured' to heaven before the end of the world.

This was merely the tip of the iceberg of his 'dispensationalist' theology, which proclaimed God deals with humanity in different ways throughout time. Dispensationalism was popularised in America via the Scofield Reference Bible. Rapture theology is now incredibly popular, despite being a recent invention. Into this febrile environment, predictions of the imminent return of Jesus are given vast amounts of attention.

The Bible itself is remarkably quiet on the issue. Although Scripture offers some hints as to when and how Jesus might return, there are no definitive answers. Matthew 24 sees Jesus proclaim that, "about that day and hour no one knows, neither the angels of heaven, nor the Son, but only the Father." However, many theologians argue this passage primarily refers to the destruction of Temple and Jerusalem in AD 70, rather than the Second Coming. Treating the Bible as if it's a fortune teller is to profoundly misread it. It isn't giving us precise dates and times and locations for what will happen in the future. Of course Daniel, Revelation and other apocolyptic literature is an important part of the Biblical canon, but we must read it with care, as evangelical theologian Ian Paul writes here.

Even if end times predictions turn out to be incorrect they're harmless enough, though, aren't they? Well, no.

First, these proclamations of Jesus' return inevitably end in disappointment. That disappointment could lead to many people who have a genuine faith in Jesus falling away.

Second, history shows an unfulfilled prediction of the end times can lead to heretical groups springing up. The groundwork for the development of the Jehovah's Witnesses was laid by the 'Great Disappointment' of 1844, when Baptist preacher William Miller had predicted the return of Jesus, for example.

Another reason end times obsession is a bad idea is that over-interpreting contemporary geopolitical events does more than just make us look silly – it makes the Gospel look untrustworthy. During the Cold War, some Christians thought various Soviet leaders were 'the antichrist.' After the fall of the USSR, those predictions looked foolish. Some have focused on Saddam Hussein, Osama Bin Laden or even Barack Obama as the antichrist. Time and again, these absurd proclamations have been proven to be what they are – nonsense. The Gospel has surely been harmed by these outlandish statements – we should run away from such speculation.

The final reason to be wary of a preoccupation with the end times is that it takes away focus from what we should be doing. Wouldn't it be better for Christians to forget all the guesswork and decoding of the things which Jesus didn't reveal to us, and instead spend our time carrying out the instructions that He did make clear? Carrying out the Great Commission and following the Great Commandment is a far more Christian way of spending our days than speculating on the latest arcane interpretation of Scripture.