His idea was to make this trust explicit. And that's the birth of Ripple. Ripple's consensus process is predicated on trust that other entities are not colluding against you.
The Ripple protocol is not subject to the 51% attack that Bitcoin is subject to. Making entries in the ledger has nothing to do with computational power.
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Link to the 2011 discussion that spawned Ripple: https://bitcointalk.org/index.php?topic=10193.msg146250#msg1...
EDIT: Adding a link explaining Ripple's consensus process for making changes to a shared ledger: https://ripple.com/wiki/Consensus