(Joseph Williams uses this proverb in his book Style.)
When I make a judgment on an article for a peer-reviewed journal, if the first paragraph is excellent, clear, and well-written, then I know immediately that the article will probably be an acceptance or a revise-and-resubmit. Why? Because the introduction is the hardest part to write. If the scholar is able to pull that off, then I can predict she will be able to write an entire article, or be competent to correct problems I see in the remainder of the article.
Conversely, if the introduction contains freshman-level errors of composition, does not give me a clear idea of what the paper is going to be about; if the thesis is weak; then I suspect the article will be a rejection. Why? Because the introduction is the part of the paper that the writer should have written with the greatest care.
In either case, the quality of the introduction has about 95% predictive value.
Writing a good introduction means that you've gone half-way toward convincing the reader that you are a competent scholar. It should also give you the confidence to proceed, having the same rhetorical effect on yourself.