How do I tell my family that I need to seek a professional help for my mental condition?

My family (mom and dad) always thought that I'm perfectly normal but I've been dealing with anxiety since I was 13 and I was diagnosed with BPD and major depressive disorder by the age of 15 (I am now 21 years old, and I went to a psychiatrist by myself back then because I didn't live with my parents). But recently, I think my mom has noticed that something is wrong with me and I'm really afraid to tell her.

My mom and dad are the type of parents who don't believe in mental illnesses, they think thay people with mental illnesses are just not close enough to God. My younger sister was caught self harming by my mom and she just told my sister to pray more. My sister begged my mom to book an appointment with a therapist. My mom booked the appointment though, but she insisted both the therapist and my sister that she wanted to join the consultation session just for her to listen what my sister said to the therapist.

Although I don't live with my parents now, but they often check up on me and they have been noticing that I couldn't do basic tasks and sometimes burst out crying in the middle of conversations ever since my mental condition is getting bad. I don't want my parents know about my mental condition if in the end they just suggest me to seek forgiveness from God.