I've never found any Colin Hanks performance to be very interesting but I haven't seen him in much. If people thought Johnny Lee Miller was blah after John Lithgow selling the insane character stylings of Trinity, wait til they bitch about Hanks.
Olmos of course is an excellent actor but the stink of BSG is awful hard to ignore. It took repeated episodes of The Closer before it wore off Mary McDonnell.
That said, in all probability the "religion" in Dexter will bear about the same relation to real religion as Jonny Lee Miller's character did to real motivational speakers. Dexter's magical abilities to spot murderers, flawlessly kill them and wriggle out of impossible situations except when the script requires a little temporary "jeopardy" long ago approached heights suggesting divine intervention. Dissolving all the hopelessly knotted mess of plot holes, bizarre characters "arcs," thematic reversals into "GOD did it!" has worked for other series. Who better than EJO to sell this cure?