I'm going to tell my friend that people who believe in astrology are deluded.
Why do you think that the other person is wrong?
Because the person is stupid and believes in star signs.
Even if you don't personally believe in astrology and find it to be asinine, it's rude to belittle others for their beliefs. You should try to be more accepting of others even if you don't support their personal choices as it is important characteristic of being a good friend.
I just don't believe in things that are not real.
You have a right to decide what to believe but you need to understand that others have that same right.