My professor, good friend and mentor always says this (which is a reference to someone else - forgot who):
"Think you're a great writer and you may become a good one; think you're a good writer and you may not become one at all."
Now I would make a modification to that. "Think you will be a great writer and you may become a good one; think you will be a good writer and you may not become one at all."
Writing, just like life, is a journey. So there are no absolutes. Writing means growing as a person, a thinker, a deep reader of literature, and of course as a practicing writer. Which is to say, you may or may not be a great writer now, but you ought to believe you will be one. If you believe you are a great writer currently, there may be people who disagree. And if you are considered a great writer currently by many, there are always higher heights to reach. So what's important is having our eyes set on the great heights ahead. But the message in these quotes would be to not aim realistically.
Aim high, aim the highest you can imagine. The highest potential as a person, as a student, as a teacher, as a writer, all in one. Be the next Homer or Ovid or Dante or Shakespeare or Blake or Hemingway or Fitzgerald or T.S. Eliot or Joyce or Kafka or Dickinson or Bronte or Tolkien or David Foster Wallace or onwards and go down in history and be studied and revered for centuries to come. Be part of social change, spark new movements and develop the human consciousness. Don't aim to be a quick trendy sell out success; that isn't aiming for the greatest greatness.
Strive for the best, and you'll get somewhere significant.