Faith is, by all definition, an opinion, but there are limits to how far those opinions can be expressed. In the private/ personal sphere, go nuts. In the public policy making sphere however, all opinions, even faith-based ones, are never absolute.
Constitutionally speaking, separation of church and state is not mandated or absolute, but is strongly implied textually and backed up as such by SCOTUS. A lot of people forget that the religious freedom clause in the 1st amendment is two fold - 1) congress shall make no law respecting an establishment of religion and 2) Congress shall not prohibit the free exercise thereof.
One prohibits government from establishing by law or establishing de-facto, any religions and the other tells government they should give religions leeway and balancing the two has been one of the most controversial constitutional and political hot-buttons. As a result, religions are protected under the free exercise clause and receive exemptions from many regulations--for example, with regards to property taxes, zoning issues, conforming use, minimum age to drink w/ regards to sacramental wine, etc. And the constitution only says 'CONGRESS' shall make no law, not the state legislatures and this combined with the fact that the 10th amendment reserves all unstated rights to the people and the STATES, state governments have, theoretically speaking, a lot of leeway.
But even with all this in mind, Santorum's statement that religion somehow has a legitimate place in the public policy making is insane and goes against an entire history of SCOTUS rulings and even elementary understanding of our constitution. I'm surprised that for a party that screams SLIPPERY SLOPE when it comes to gay-marriage, that they don't see religion mixing with policy making as the biggest slippery slope of all and a tremendous danger to all -- to government, religion and the people.