Arguably, a deity with sufficient knowledge and power would know exactly what is required to convince me and would be able to do so. Or, if He gave you free will, he would give you the opportunity to open the door when He knocked.
All I can say is that, based on personal experience, He does knock.
Well, if He does knock and He is capable, then He will let me know when He wants me to know. Until then I have no more reason to believe that He exists than I do Zeus or Odin.