I have created login and registration pages styled how I want them to look on my website. I am using Contact Form 7 as the fields for my forms to sign the user up and log them in. How do I make it so that when a user submits the login form, it logs them into my website?

I have tried using a JavaScript file that is triggered upon submitting the form to take the values and input them into the default WordPress page, but I feel like this could be a security issue. It also was not working 100% of the time. Closer to approximately 25% of the time.