The mining of any given block of data is difficult to accomplish, but once the work has been done, it is extremely easy to be validated by other nodes in the network. In short, this is how the consensus mechanism functions. If the majority of nodes in the network agree to the previously mined blocks, they will keep adding new blocks on top of the old ones, forming a longer chain over time. The longer the chain, or in other words, the more validations previous blocks have, the safer any given block is to be truly valid.