Why does Blake ask the salesmen if they have "Made your decision for Christ"?

I believe he was delivering a paraphrased "Come to Jesus moment". It is defined as:

...the "concept" of the moment at which you decide to accept Jesus Christ as your Lord and Savior, but it has a secular meaning. A "come to Jesus moment" refers to a dawning, epiphany or agreement following a disagreement. It refers to when the light blub comes on and you understand something or make a critical decision.

The decision they were making was to sell the hardest they could, leaving mediocrity behind. Also, they were clearly fighting for their continued existence within the company.

The answers are accurate but take the point of view that Blake was asking this of the salesmen. He wasn't - it was a rhetorical question in the context of AIDA, which is what he was describing to the salesmen to further humiliate them (AIDA is Sales 101).

He's asking them to ask the prospect to make a decision as if they are committing themselves to Christ - absolute, genuine, irrevocable.

You see this in Ricky Roma's pitch later in the movie to the uncertain buyer, about faith, and then when Ricky derides Kevin Spacey's sale to the gentleman who signs contracts and then backs out of them ("Vishnu himself could come down and say sign and he still wouldn't go through with it!")

I think it is intentional and the intention is to echo the moment in revivalist religious meeting when the evangelist gets the waverers in the congregation to commit to God. Whether you are a believer or not, this is the moment where the fervour has been whipped up in the meeting and the potential converts are in an emotional enough state to make a public commitment to change and repentance so their behaviour will, in future, be different.

Since many of the techniques he uses are a pretty good psychological analogy to those used by revivalist preachers, the line is likely intentional. I doubt, given his obvious character, that it has anything to do with his personal beliefs. He is merely aping the often shysterism psychological manipulation of many preachers and snake-oil salesmen. The line merely emphasises this link.

David Mamet has never written a line he didn't intend. Blake's speech is not motivational, it is a terror-tactic. What he's saying is that he's firing all but two of the sales staff, and since Ricky isn't there at the time it means only one of the salesmen he's talking to will still have a job when the "contest" is over. The decision he is referring to is not the salesmen's decision, it is the decision of the customer. He's saying in order to sell anything, you need to lead the customer to commit fully to the sale. When he says "Have you made your decision for Christ?" he is modeling sales technique in the most brutal, cutthroat way possible. The salesmen must inspire that kind of commitment in their marks if they want to stay alive.

Interestingly enough, the Blake character and scene do not exist in the original play. Mamet added it to the movie to raise the stakes of the story, which he most certainly did.