There are two Bitcoins:
Bitcoin protocol: The technology that makes up the protocol.
Bitcoin blockchain: The most popular implementation of the technology.
The Bitcoin protocol uses cryptographic technology that has not yet been hacked. (If it were hacked, there are much bigger problems to worry about).
The Bitcoin blockchain relies on a network of operators to make sure the blockchain/ledger/database tracking BTC accounts and balances is current and reliable.
What makes the Bitcoin blockchain theoretically reliable is that no single operator can make unilateral changes to the database. To do so would require controlling a majority of the computing power used to validate changes to the database.