This is an interesting problem I stumbled upon today involving (I think) respect. Compare the following sentences:

"Your dinner is ready, my lord."
"Yes, my lady."
"As you wish, lord."
"It shall be done, lady."
"Lord Arnaught wishes to speak with you at once."
"The lord of castle constance awaits your pleasure."
He looked up and saw the lord standing over him.
He looked up and saw the lady standing over him.
He looked up and saw Lord Arnaught standing over him.
He looked up and saw the Lord standing over him.

See, if your lord is suddenly nameless he becomes "the lord." Just like 'the duke' or 'the earl' or, as in my examples 'the lady.' The problem? Lord or The Lord is usually a specific title given to Christ. So when you read "He looked up and saw the lord standing over him" without context, are we talking about some random lord or "The Lord"? What about capitalization? Does that make a difference? Is it simply cultural usage and not something to worry about? Or is the coincidence to much to ignore and we should always use 'my lord'?

Generally, capitalizing it makes it refer to God. The only time you capitalize otherwise is if it is placed as a title, in which instance it is clear that it refers to that person. If it is in all caps, it refers to a specific name of God, Yahweh.

Culturally people in medieval timing would have used the term ‘my Lord’ as a title, so it would have been capitalized.

Yeah, that was sort of what I was trying to say, if it refers to a person and is a title it would be capitalized.

I don't think there's anything wrong with using "my Lord", since it's a cultural thing.

In Spanish, it is not uncommon to call Jesus, "Senor Jesu Cristo."

It is also not uncommon to call a mere human "Senor Smith."

I feel like this is much the same thing.

Yes, exactly like that, Willow. Thanks.

Finally, someone gets what I'm talking about.

I think for me it is rather easy for people to distinguish when I am referring to THE Lord and when I am referring to a Lord. I generally never use Lord in an actual non-dialog paragraph unless I am referring to the Lord of all. I keep it as 'him' or 'her' or 'the man' or 'the woman.' Because, technically, when talking to someone in dialog that is referred to as Lord or Lady, you put a comma before saying it. When you are talking about God, you do not.

Lord as a person:
"Will you have some coffee with me, my Lord?"

Lord as God:
"Will you have some coffee with me my Lord?"

It is a slight difference that most people pick up on naturally actually, besides the obvious pointing of context. Also, when you are writing simple paragraphs from a third person perspective or if you are in a person's thoughts, using that title of respect is highly unnecessary because it takes away from your characterization. You want to character your actual character as being respectful, not the author. It would do little but throw off your readers. Thus, all titles are best if kept primarily to dialog.