Correct me if I'm wrong but isn't there something about following Him or repentance or something?
Just like all of the commandments are wrapped up in "loving God with all your heart and soul and mind and Loving your neighbor as yourself", believing on Jesus does imply that you keep his commandments. Or at least make a gallant attempt.
But the promise of God is the promise of God. If He doesn't keep his word, then can any of us be assured of salvation, even if we are "perfect even as our Father in heaven is perfect?" No. Our salvation is not determined by our systematic theology. It is determined by the promises of God. Period. John 3:16.
You want to argue with Jesus?