I can see both sides of this.
You planned a wedding which suited you and DH perfectly. I agree that you shouldn't feel obliged to have your wedding any other way to suit other people. And I can understand that you wanted to keep it as small as possible as that suited both you and your own parents best.
But I can also see your IL's point of view. they hold strong religious views, and possibly have dreamed for many years about seeing their son being married in the eyes of God and all that.
They are now dissappointed and probably don't understand why other people were invited, yet they weren't. I totally understand your reason for inviting friends to be witnesses rather than strangers (a very sensible arrangement, in my view). But as close family, I can understand their hurt.
And, from an outsider, it seems as though you planned your wedding as a small affair, with minimal pomp and ceremony. Yet now you want all the congratulations and fuss. You are intending to have a big party and mini-celebrations. Which seems to contradict the arguments for having such a small and intimate ceremony in the first place. You are the mother to their grandchildren, so presume they already treat you as part of the family, so don't understand why you are expecting to be welcomed into the family?
You refer several times to threats of not speaking to you ever again. but it seems as though this was some time ago and they have since spoken to your DH. So it sort of feels like you are still resentful about this and their unreasonable initial reaction to your news. You have reached a stand-off.
So where do you go from here? I think as it was you and your DH's actions that have caused this situation, it should be you who blinks first and resolves the situation. Please, pick up the phone and speak to them yourself. While its just been their son who has spoken directly to them, its quite easy for them to blame you in their minds for leading their DS down a path which he didn't necessarily want. I'm sure it was a joint decision how to do your wedding, but until YOU make contact with them (directly, not by phone, letter, text, email etc), they can keep viewing you as the 'bad' one.
You had the wedding you wanted (and I can see why that suited). They are upset and have over-reacted. But make peace now, this isn't worth a family rift.