I think the best sort of way God could reveal himself would be as a constant Spirit. Comparable to how God was with Adam and Eve. Walking with them and talking to them. God as a close and loving Father, rather than a distant loud voice.
The possible doubt would be about whether advanced aliens could doing this to us as an experiment. <-I admit, That's funny!
The problem might not be that atheists can't come up with a doubtless way of knowing God is real, but rather God is just the sort of thing that can't be known without doubt. God isn't physical so we can't never see God as he is... there is nothing to see. We, being physical, can only be influenced by physical things. So any limited physical expression of God could be copied by advanced technology.
But Jesus was physical and He is God... and there is evidence that He existed and was seen by many multitudes.
Jesus is a theophany (God in the human form) and appearance of God. He states "I AM the father" "I and the father are ONE." "Why do ye ask' show us the father? For when you see me, you see the father." He states "I have all the power on heaven and on earth" Matthew...
(1) In Zechariah 12:10, God speaks of the future return of Christ and says,
". . . They shall look upon me whom they have pierced, and they shall mourne for him, as one mourneth for his only son . . . ." This is spoken again in Revelation 1:7, which is clearly a reference to Christ Himself, but in Zechariah 12:10
GOD said,
". . . They shall look upon me whom they have pierced . . . ." The One that they pierced was Jesus Christ! So, in Zechariah 12:10, God is saying that He is Jesus Christ.
(2) In John 1:1, John 1:14, 1 John 1:1, and in 1 John 5:7, Jesus is called
"the Word," and John 1:1 says that
"the Word was God."
(3) Thomas referred to Jesus as
"My Lord and my God" in John 20:28, and Jesus did not see the need to correct him.
(4) Isaiah 7:14 gives us the prophecy of the Virgin Birth of Christ and states that His name would be
"Immanuel." Matthew 1:23 tells us that this word means
"God with us."
(5) In Isaiah 9:6, Jesus is called
"Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace." The Bible certainly would not say this about anyone less than God.
(6) In John 10:30, Jesus says,
"I and my Father are one." Jesus is not a lesser god; He is ONE with the Father.
(7) Micah 5:2 tells us that Jesus is
"from everlasting."
(8) In John 8:58, Jesus tells the Pharisees,
"Before Abraham was, I am." He claimed to be the
"I AM" of Exodus 3:14, which is God Almighty.
(9) Jesus allowed others to worship Him (John 9:38; Matt. 14:33; and Luke 24:52), which was forbidden, unless He was God (Rev. 22:9).
(10) Jesus forgave sins (Mark 2:5), which only God can do (Mark 2:7).
(11) The Lord Jesus Christ is omnipresent (Matt. 18:20; 28:20; II Cor. 13:14; I John 5:7). We know He is God because He is capable of being everywhere at once.
(12) Jesus is omniscient (Mark 11:2-6; Matt. 12:40). He is God because He knows all things.
(13) The Lord Jesus is also omnipotent (Rev. 19:6; Matt. 28:18). He has all power.
(14) According to John 1, 1 John 1, Colossians 1, and Hebrews 1, Jesus Christ is the Creator of Genesis 1!
(15) Jesus never sinned! Romans 3:23 says that ALL HAVE SINNED and come short of the glory of God, but Jesus did not sin (2 Cor. 5:21; 1 Peter 3:18). So Jesus had to be God.
Jesus also states "Neither be ye called masters, for ONE is your master, Christ"... Then God is referred to as the one master "You cannot serve two masters, you either love or hate the other... You cannot serve both God and mammon" meaning Jesus IS God, the ONE master (no humans or things).
JESUS IS GOD!
So the best thing would be for God to show himself as a ever present Spirit, or many present Spirits... a manifestation of God for each individual. Still we would have to trust the claims of this powerful, wise, and knowledgeable Spirit.
But God showed Himself in Jesus, who was in the flesh. "God is the word and the word became flesh" John.
John 1:1 - In the beginning was the Word, and the Word was with God, and the Word was God...
1:14 - And the Word became flesh and dwelt among us,
Isaiah 44:6 - (God is the Redeemer)
2 Peter 1:1 (Jesus is the Redeemer) - To those who have obtained like precious faith with us by the righteousness of our
God and Savior Jesus Christ
Titus 2:13 - looking for the blessed hope and glorious appearing of our great
God and Savior Jesus Christ
Is this a problem for atheists, or an inherent problem of metaphysics? Maybe there can be no clear evidence for metaphysical claims. It wouldn't be our problem, it is the theists making the claim.
Something to think about: atoms cannot be seen, and centuries ago, their presence was based merely off intuition without any evidence. "Seeing" is not always believing.
Science proven to work much better than speculation about gods and spirits. If there is a problem with the scientific explanation it will become more clear as we better understand the science around the issue. Consciousness could be one such subject. But it is hard to know whether consciousness can be explain by science or not until the brain has been better understood.
But Jesus cannot be speculated as He actually existed. Even old churches in the medieval age may have had relics from His cross. And JESUS IS GOD. Proof?
"11 Believe me when I say that I am in the Father and the Father is in me; or at least believe on the evidence of the works themselves."
Jesus's God's words, John 14:11
I would consider it very strange if God hid evidence of his existence so deep within the universe. One would think that God would want uneducated farmers 3000 years ago to believe in Him, not just people after the year 2050 AD.
His evidence is in JESUS, God in the human form given as a theophany, an example to follow. As He had appeared to Moses - Moses saw God's back after He saw Him in the bush, and Jacob wrestled with God in the flesh and saw Him "face to face." Adam and Eve saw God when He walked in the Garden.
Well there probably can't be scientific evidence of a non-physical being.
But there is evidence of JESUS, who was God in the flesh as a theophany (appearence of God) as seen in the past.
But if we currently have no evidence of God (or no understanding of this evidence) then we have no reason to say there is a God. Maybe there could be evidence for God, and maybe there could be evidence for invisible immaterial unicorns in my room, but until there is evidence I don't think such unicorns exist.
JESUS IS GOD. There is evidence He was there.
Well he could probably communicate, but that doesn't mean we could know it was Him.
You know because all good comes from God, All good comes from Jesus. You know from the catacombs that people saw Him and what He did. Evidence from 30 A.D. on ward to 313 A.D. JESUS IS GOD, GOD'S THEOPHANY to give us a PERFECT, sinless example to follow.