I was visiting a bird-supplies store when I mentioned to the owner that my wife and I had owned a hummingbird feeder but had never once seen a hummer at the feeder, so we tossed it out. I concluded that there were no hummers near our home.

The shop owner asked where we lived, I told him, and then he said, 'They are there. Not only do some of your neighbors have hummers on their feeders, but hummers are all over the village.' What he said next was the take-home line: 'You just have to have eyes to see them. Once you do, you will see them everywhere. They are small and fast and camouflaged, but they are not that hard to spot.'

Eventually we bought a new feeder, filled it, and waited until our eyes got accustomed to the sight of hummers. Kris and I now see them everywhere. When other people go on a walk with us, we often observe a hummer – but it is rare that our friends spot one. It takes experience. You need to learn to spot them out of the side of your eyes and acclimate to their habits of zooming and darting and taking shelter on obscure branches and even on telephone lines. But once you've learned to spot a hummer you will see them everywhere because they are everywhere.

Like angels. They, too, are all around us. Few of us have seen one because we first have to learn what we are looking for. In a good book about angels, Martin Israel, quoting a friend, wrote this: 'Eternity lies all round us and only a veil prevents us seeing it.'

The hum of angels surrounds us, and we only need ears to hear it or eyes to see them. Or perhaps a special sense for them. After all, the Bible tells us that Balaam's donkey could see an angel that Balaam himself could not see. The passage suggests that Balaam was so insensitive to God that he failed to notice. At the end of her survey of angels, Jane Williams, author of an important book on angels, invites us to hear and join in the chorus:

'Behind, around, underneath, and through the day-to-day world that we inhabit is the song of the angels. It is beautiful, endless, joyful, and terrible. It will be sung whether we join in with it or not, but imagine the sensation of stepping into that angelic harmony and being caught up in its power and majesty. This is what the angels invite us to do. They long to teach us their song, so that we, with them, can sing a hymn of praise to the glorious universe and its maker.'

It should not bother us that dimensions of reality exist that we cannot see. Dogs can hear sounds we cannot hear; birds can migrate up and down the globe with a sense of direction that far exceeds our own; those little hummingbirds somehow know when to head north and when to head south, and all of a sudden they disappear from our summer feeders. Their sense of timing is unerring. Squirrels can somehow smell or feel nuts planted underground for winter storage; and bees... well, bees are a world unto themselves.

And those examples are just a few of the creatures living on earth. God knows vastly more than we do; God sees and senses vastly deeper than do we. So why should we think that only what we see, hear, or sense is all there is? There is a popular conviction that we can believe only in what we see and feel. The best word for this posture is arrogance. Bold and not beautiful.

Kris was driving through a wooded area the other day on the way to her office when a deer, seeming to suddenly notice her, stopped just before running into her lane of traffic. Had it continued on its initial path, Kris could have been in a serious accident. The timing was right for it to have happened. But she called me later and said the deer's moves were so unusual she wondered if she had been protected by an angel.

As it is with spotting hummingbirds, experience is our teacher. Kris's story reminds me of another one. At the time of this previous experience, we did not consider the involvement of an angel. But on further reflection perhaps we should have. (Jet lag might have prevented us from thinking this way at the time.)

We were on vacation in Italy. After landing in Milan in the north, we drove east toward Venice and had as our first night a stop in Verona. Having arrived in Verona, we quickly realized the city seemed like circles inside circles. We asked someone for help, and he pointed us forward, but after just one curve we were lost again. And jet-lagged.

Suddenly a kind man appeared at our car window. I showed him the name of our hotel, and he said, 'Can I get in your car? I'll direct you to the hotel.' Which he did, and we were overjoyed. We got out of our car and turned to thank him, but he seemed to disappear with a shuffle of his feet and a walk down the street. Was that man the hum of an angel? Perhaps so. Angels are everywhere, but you have to have an ear tuned to their hum, an ear sensitive enough to be surprised by angels.

Here are a few more things we learned from the gentleman at the bird-supplies store, who is a walking hummingbird encyclopedia. These brilliantly designed birds can fly in all directions and they consume thirty times their weight each day. They get into aerial combat with one another over who controls which feeders in the neighborhood. The man at the shop has studied data accumulated over centuries, and he speaks authoritatively and accurately.

The same can be said about those who seek to accumulate knowledge about angels. One angel passage in the Bible will give us something, a bit like Kris's and my limited experience with ruby-throated hummingbirds at our backyard feeder. (We've never seen any other kind of hummer there, but dozens of kinds of hummers populate the world.) The same is true with learning about angels. But if we look carefully at all the passages in the Bible that speak of or about angels and then read up on what theologians and historians have said, we can know plenty about them.

***

Here is my claim: 'If you believe in God, you also believe in angels.' This is not a philosophical book about angels, nor is it an attempt to prove the reality of angels. I'm thankful the philosophers and apologists have set forth persuasive arguments in favor of angels and so have cleared the way for me to talk about what the Bible says about angels. I will do just that on the basis that belief in angels is intellectually credible.

Instead of my attempting to prove the reality of angels, I invite you to dig in with me to see what the Bible says about angels. Here is a foretaste: there are far more angels-related passages in the Bible than most Christians know. The Bible, we will see, describes this world as full of wondrous beings, many beyond the scope of our eyes.

Scot McKnight is Professor of New Testament at Northern Baptist Theological Seminary and blogs at Patheos.

This is an extract used with permission from 'The Hum of Angels: Listening for the Messengers of God Around Us', published by Monarch.