Gain ground upon truth and reason

Could explain " how fast mistake and prejudice, when neglected, gain ground upon truth and reason" for me?

Is it like "A thousand repeats of a lie would make it as a truth"? Since the mistake and prejudice prevail, so they will become 'truth"?


Context:
I wish, also, to intercept and suppress those prejudices which particularly prevail when the mechanism of painting is come to its perfection, and which when they do prevail are certain to prevail to the utter destruction of the higher and more valuable parts of this literate and liberal profession.
These two have been my principal purposes; they are still as much my concern as ever; and if I repeat my own ideas on the subject, you who know how fast mistake and prejudice, when neglected, gain ground upon truth and reason, will easily excuse me. I only attempt to set the same thing in the greatest variety of lights.

I've read your reply twice and still didn't get it clearly.

I think I will read it again later.