Complex sentence: Bonisagus, Guernicus, Mercere, and Tremere Houses are the True Lineages, which means that each magus was taught by a magus of that House, and that master-apprentice relationship goes back to their founders.
Split into simpler sentences: Bonisagus, Guernicus, Mercere, and Tremere Houses are the True Lineages. This means that each Magus was taught by a Magus of that House, and that master-apprentice relationship goes back to their founders.