Ok, Dave's right, blogging's about the quick post. So here goes...
The Ten Commandments are written in the indicative. Did you know that? There's a perfectly straightforward imperative mood in Hebrew. God could easily have said "You must not murder". But God didn't say that. He said "You will not murder." You won't. You're my special people. I've saved you. You won't lie, you won't murder, you won't covet. You won't. These things are not said in the (grammatical) mood of command. They are said in the mood of promise!
Now of course they carry commanding force. When a mother says to two screaming kids "There will be peace in this house", by golly there had better be peace. And when God says there will be peace, well there's a huge commanding force to that. But it's first and foremost a promise.
And because it's a promise, it becomes the most binding command.
"You will" is far stronger than "you must".
"You must" implies that you may not. "You must" puts you in the driving seat. To be sure it stands above you with a threatening tone. But even after "You must" is spoken the reality is that maybe you will and maybe you won't. The choice remains yours.
"You will" takes the choice out of your hands. "You will" does not even contemplate an alternative. "You will" binds you to the promise. It makes you a slave of grace. It casts you as a humble recipient of the word with nothing to do but walk in the service that is perfect freedom.
So now Jesus says this in Matthew 5:48 - and again, He could have used the imperative. Instead He spoke in the glorious future indicative:
You will be perfect as your heavenly Father is perfect.
What a command? Well, yes, subsequently. But first - what a promise!