How does consensus protect from lying about transactions?
You can think of a bitcoin address like a machine that produces locks and a bitcoin wallet like a machine that produces unforgeable, labeled keys.

Say Alice received 1 bitcoin from Bob and wants to send it to Charlie. If Alice has 1 bitcoin to send, that means there is 1 bitcoin that is locked with a lock from her lock-making machine.

When Alice wants to send that 1 bitcoin to Charlie, she must have his address. She uses his address as a lock-making machine and makes a lock that only Charlie can unlock. She feeds this to her key-making machine and makes a key to the lock that only she can make a key to but labels it with the lock she got from Charlie's lock making machine.

This new key only opens the 1 bitcoin Alice will send to Charlie, and only Alice could make it. The attached new lock can only be opened by Charlie, since only he has the matching key-making machine.

You consider someone to have "sent you a bitcoin" if they locked it with a lock that only you can open.

So, a block that contains invalid transactions is itself invalid and ignored. You can only form valid transactions that transfer bitcoins with the appropriate keys or the transactions are invalid.

This might make you wonder what you need consensus for at all. And the problem is that Alice could maliciously make multiple keys that open the same lock, transferring the same 1 bitcoin to both Charlie and Dave. We need consensus to agree which of a set of conflicting transaction, each individually valid, we can rely on.