Apple's iMessage Is Secure ... Unless You Have iCloud Enabled
www.howtogeek.com
Apple's iMessage isn't as private as you might think. There's a big hole in Apple's end-to-end encryption scheme, and it's called iCloud.

By default, iCloud backups are (1) enabled and (2) unencrypted

You would have to get not only yourself and every person you’re chatting with to manually enable encryption for their iCloud backups on their iPhones, but also every device they have that is using iMessage