Take a look at that last phrase: "who through faith are shielded by God’s power." Just one verse later, Peter acknowledges these Christians have “had to suffer grief in all kinds of trials” (v. 6). So here’s the million dollar question: what, exactly, does God’s power shield us from? The answer can’t be “grief in all kinds of trials.” So what are we being shielded from?
 
This section in 1 Peter focuses primarily on the inheritance that awaits believers. Peter directs our attention towards the future; towards eternity. So maybe God’s shielding is related to that. Let me illustrate:
 
Imagine a friend comes you one day and says, “I want to take you to an oceanside paradise."
 
After showing you pictures, you reply, “Great! When do we leave?”
 
He goes on to explain that it’s going to take five years to get there and he’s not sure exactly how you’re going to get there, but he’s absolutely certain the two of you will end up there eventually. It sounds a bit dubious to you, but you agree because the paradise that awaits you looks worth it and your friend is trustworthy and so you begin your trek. The first stage is driving where for the first couple of days you make good progress, but on day 3, the car breaks down in the middle of nowhere. You call a tow truck to come tow your car to the nearest mechanic who then tells you, you blew a head gasket which caused one of the pistons to seize up inside one of the cylinders and the engine is now trashed. So instead of trying to fix the car, you decide to rent a car. You travel another day, but while stopped at a rest area the two of you are assaulted, robbed, and your car is stolen. Now you have no transportation and you’ve run out of money. So you resort to hitchhiking which works for about 60 miles until the police catch up with the driver you’ve decide to tag along with. Turns out there’s a warrant for his arrest. While you don’t face any charges, his vehicle is impounded and now you and your friend are on your own trying to figure out your transportation needs. You end up spending the night under the stars on more than one occasion. Throughout the duration of your five year long journey, you face one obstacle after another: physical injuries, sickness, financial loss, conflict with your friend, but you never give up. Why? You want the paradise that awaits you. 
 
When we’re told God uses our faith like a shield, that doesn’t mean our faith protects us from bad things happening to us. Our faith shields us from a greater threat. In your harrowing journey with your friend, your faith never prevents the car from breaking down nor protects you from injury or financial loss. God uses our faith to shield us from a greater threat than the trials of this life. God uses our faith to shield us from the one and only catastrophic threat we all face: giving up.  
 
So my prayer isn’t so much: “protect me from trials.” My prayer is: “when I do face trials, protect me from throwing in the towel."

Why do we face trials? The Bible has numerous ways of answering that question. Sometimes it's because God is exercising fatherly discipline (Hebrews 12). Sometimes it's due to consequences for sin (John 5:1-15). Sometimes it's because God wants to display His works through the hardship we're facing (John 9). Regardless, all trials test the authenticity of our faith. 

But for whom?

God already knows whether or not your faith is real. It's not as if He isn't sure and has to send some hardship your way so He can find out. God doesn't sanction the trials we face so He'll know if our faith is legit. He sanctions the trials we face so we'll know if our faith is legit.

Suffering is a crucible. Through it, God says, "Now we're going to find out whether you got into this relationship with Me to serve Me or to get Me to serve you." The way we respond to trials either authenticates real faith or exposes our "faith" as fraudulent.

The 1 Peter passage ends this way...

May it be so!