Regardless of whether or not you provide your own SSL certificates, cloudflare still uses their own between their servers and client browsers. So any SSL encrypted traffic is unencrypted at their end before being re-encrypted with your certificate. How can such an entity be trusted?
CF provides different encryption modes. So if it’s “Full” you’ll need a valid SSL cert on your server, which CF will use end-to-end. If it’s “Flexible” (IIRC), then you don’t need a cert on your server, in which case CF will use their own cert for encryption.