Your title says it accurately: Google Safe Browsing flagged your website.
Your website was flagged by Google Safe Browsing, which means only Google Safe Browsing can get your website removed. You don’t need to convince us that your website is safe, because we’re not Google, and so we cannot help you with this.
I can tell you from experience that Google seems to deeply distrust websites that only have a login page, and is likely to flag them as deceptive, presumably because they consider it to likely be a phishing page. Having a landing page with your website that provides some introductory content tends to help a lot.
And if you’re not that far yet, then adding password protection (through the Password Protected Directories feature) or making it so that only your own IP address is allowed to access the website should help prevent Google from flagging your site.
Until then, all you can do is ask Google to review your website. That can be done through Google Search Console.