Despite being absolutely public, or rather because of that fact, Bitcoin is extremely resistant to tampering. A bitcoin has no physical presence, so you can’t protect it by locking it in a safe or burying it in the woods. In theory, all a thief would need to do to take it from you would be to add a line to the ledger that translates to “you paid me everything you have.”
A related worry is double-spending. If a bad actor could spend some bitcoin, then spend it again, confidence in the currency’s value would quickly evaporate. To achieve a double-spend, the bad actor would need to make up 51% of the mining power of Bitcoin. The larger the Bitcoin network grows, the less realistic this becomes as the computing power required would be astronomical and extremely expensive.
To further prevent either from happening, you need trust. In this case, the accustomed solution with traditional currency would be to transact through a central, neutral arbiter such as a bank. Bitcoin has made that unnecessary, however. (It is probably no coincidence that Nakamoto’s original description was published in October 2008, when trust in banks was at a multigenerational low.) Rather than having a reliable authority to keep the ledger and preside over the network, the Bitcoin network is decentralized. Everyone keeps an eye on everyone else.
No one needs to know or trust anyone in particular in order for the system to operate correctly. Assuming everything is working as intended, the cryptographic protocols ensure that each block of transactions is bolted onto the last in a long, transparent, and immutable chain.