A Question about Penal Substitution

So, in the wake of all the Love Wins kerfuffle, I received an email from someone who listened to the radio interview I did with Michael Horton.  And the question he asked was this: Are penal substitution and universalism mutually exclusive?

Here’s how we got there.  On the interview, I asked Mike if understanding the atonement via the penal substitutionary theory was essential for a person to be considered a Christian.  He answered that yes, it is.  Other metaphors that explain the atonement are important, and even biblical, he said, but the penal substitutionary understanding is the most widely attested in scripture.  It is necessary and primary.  All other metaphors explaining the atonement take a back seat.

OK, let’s say, hypotehtically, that Mike is right about this.  Let’s say that Jesus did die as a sacrifice, to mitigate God’s wrath against every human being, wrath that was kindled because our sins of disobedience against God.

Couldn’t a penal substitutionary understanding of the atonement also be championed by univeralists?  Couldn’t a universalist affirm that Jesus did, indeed, die to take the stain of Original Sin from us, to appease God’s divine sense of judgment, and to open the gates of heaven to all people?

The obvious counter to this is that Paul said that one must believe in her heart and affirm with her lips, “Jesus is Lord” (Romans 10:9-10).  But universalists have to answer for this verse regardless of how they understand the atonement.

So, I put it to you, are penal substitution and universalism mutually exclusive?