Hypothetically speaking- even if God stands in front of you right now and say, "son, I am God" you'd still have doubt, call that person "crazy" and even if that person perform a miracle in front of you, you'll be amazed but you wouldn't accept him to be God.
Jesus-God the son, came to earth, walked and lived among men. Told them who he is and performed miracles. Many people didn't believe and called him discipline of Satan, etc..
If he came today, it'll be the same because FAITH is not based on "see first believe later" but it is-
"The substance of things hoped for, the evidence of things not seen." -Hebrew 11:1
Believer or non-believer - everyone - know that God exist- for Hmong, it's tswv ntuj (God of the Sky), Yawm Saub etc.. For Muslim -Allah (??) because God put this knowledge into the heart of every man:
"...[W]hat may be known about God is plain to them, because God has made it plain to them. For since the creation of the world God's invisible qualities--His eternal power and divine nature--have been clearly seen, being understood from what has been made so that men are without excuse" (Rom. 1:19-20)
"The fool says in his heart, `There is no God'" (Psalm 14:1)
"For every house is built by someone, but God is the builder of everything." (Heb. 4:4)
SOOOOOOOO God will not magically appear in the physical form to EVERY ONE and give you the PROOF you seek BEFORE you either believe or disbelieve.
When your parents guide you, do you expect them to show you the outcome before you accept their teaching(s)? If that's the case, can you imagine what they have to go through each and every time they teach you/guide you...
God is wise - he explain himself in his autobiography we call THE BIBLE - that book is addressed to each of us individually - and when we read, it's like reading an interview of our questions like "so God who are u?" Why am I here? Why did you created me? Why am I suffering? Are you real? How do I know you're real? etc...things like that
w/o God coming down to talk in the physical with each and every living soul from the beginning of time to the end of time -
I hope this answers your question.