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.
CF presents their OWN certificate to the client (easy to check). With “Full”, they re-encrypt the traffic with your certificate before sending it to you.
Regardless which mode you use, they decrypt the traffic with their own certificate.