The quote you ask about is spoken by Dr. Van Helsing, the vampire hunter, who speaks English with a Dutch accent as he learned English in his homeland of Holland. Not only does he have accented speech, he also has peculiarities of syntax related the Dutch variety of English he speaks. The first sentence of the quote has one of these peculiarities of syntax. I'll paraphrase it to reflect Standard British and American English, then the meaning of his words will be more clear. He says:
We have seen hitherto how good light all the little things have made.
A paraphrase of this is: We have already seen what good light of understanding all the little things, the little details, have produced [for our quest].
In the next sentence,
We have told our secrets, yet no one who has told is worse for it.
Van Helsing goes on to say that each person has shared secrets. These shared secrets, these things of which they are not so proud or things that would otherwise be kept private as confidences or intimate moments, have not resulted in any ridicule or shame or regret or any other kind of harm coming to the person who shared.
What the whole message means is that every little detail of personal experience and observation has thus far been important to the quest to resist and conquer Dracula. Therefore every little detail must continued to be shared, for none have been harmed in any way. Thus all who openly share every minute detail will continue to be safe and respected.