It's one thing to carry a cross-bearing standard into battle, or to build a crucifix with government dollars, where the money is being used to promote a given ideology.
It's quite another to purchase equipment from a manufacturer who has chosen to encode a religious reference in an inconspicuous serial number.
Until now, who routinely reads serial numbers? (I acknowledge this is non-zero - these things have to be inventoried. And perhaps the soldiers they're issued to have to sign off on them, but I wouldn't expect it goes that far.)

But let's toe this line.
What if the company wasn't named "Trijicon" but "Tanakh Optics", but aside from that change was otherwise run identically - could the Gov't still contract with them? Could their name be printed on the devices?
What if they opted to donate all proceeds from one quarter to a Christian charity?
What if they donated ALL profits to a Christian charity and worked "at cost" out of the "goodness of their hearts"?