Our deacon punted during his homily – he said he couldn’t explain the Trinity to us. He spoke about relationships instead, saying the fundamental aspect of Christianity is that we are united in faith by the Holy Spirit. I don’t think preachers should shy away from preaching about the mysteries that unite us. The fact is he didn’t have to fully explain – who could – but he could have given a whack at it I think. Read this entry in the Catholic Encyclopedia if you wish.