If they have any interest in communicating with us at all, they are likely not bounded by the 1500 year minimum. And if they are, for all we know, they could have sent a spacecraft our way 1,499 years ago upon seeing organic vegetation on our surface. I think the bottom line is that for most people, such an encounter would be very difficult to comprehend. Our culture would change rapidly and most major religions as we understand them will likely struggle to survive. And that’s assuming good intent. Even with that assumption, it will be difficult for most people with earthnocentrism to accept the new paradigm. It would be like being told the Earth is round in 1491.