You are confused! Sure, there is no need to believe something SHALL come if it has already come. That is unbelief being expressed or refusal to believe what has already been fulfilled.
However, believing IN someone is quite different. You are suggesting that when Christ comes all believers will turn into unbelievers because no one can believe IN CHRIST after he is come. When he come will you stop believing IN him???? Or the very fact that He has come will not that even strengthen your faith IN him even more?
Tell me, did not Jesus already come and live on earth. Was it impossible for those living on earth with him, seeing him every day to believe in him???? Your rationale is suggesting that being able to see Jesus denies one can believe in him.
Therefore, your analogy and rationale are wrong!
Your reply is unsound because it confused "belief in the unseen" as being the same as "belief in the seen", which are two different kinds of belief, one requires faith, the other does not.
Believing Christ was a man didn't save any of those who saw Him and heard His preaching, only believing in what they could not see with their eyes, that He is the Christ, the Son of God….saved.
The demons believe Jesus is the Christ, the Son of God because they saw its true with their own eyes THEREFORE that kind of belief that doesn't require faith in the unseen is NOT what I'm talking about. That equivocation rendered your reply irrelevant.
Its belief in the unseen Name of Jesus that He is the Christ, the Son of God, through faith which is not of our eyes, through which salvation comes:
16 "For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life.
31 but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in His name. (Joh 20:31-1 NKJ)
8 For by grace are ye saved
through faith; and that not of yourselves: it is the gift of God: (Eph 2:8 KJV)
When Christ arrives with His angels in glory, and sits down on His throne, the fact He is the Christ, the Son of God is no longer a matter of "saving faith in the unseen", then everyone knows it as fact.
Just as believing in the fact of Christ's humanity didn't save those who saw Him, neither will believing what everyone sees with their eyes---that Jesus is the Christ the Son of God, save them at His coming. That's why Matthew 25:31-46 is a judgment of works, not whether they agree its Jesus the Christ and Son of God who sits on the Throne judging them. That everyone knows, it doesn't require faith to know it, they all see it with their eyes:
Now faith is the substance of things hoped for, the evidence of things not seen (Heb 11:1 NKJ)
Therefore the dispensation of "belief requiring faith in the unseen" is now, it cannot exist after Christ arrives destroying the need for faith because then this truth is seen.
Hence I am not the one who is confused. The reasons for my position are sound.
During the millennial Kingdom people aren't saved by faith in Christ's Deity, they are judged according to their works, whether they receive Christ or not. The specified works in Matthew 25:33-46 illustrate precisely that, whether one has received Christ, ingested His teachings and lived according to His light, or not---even if they weren't aware that is what they were doing...even if they were a law unto themselves, their conscience either accusing or excusing them.
31 When the Son of man shall come in his glory, and all the holy angels with him, then shall he sit upon the throne of his glory:
32 And before him shall be gathered all nations: and he shall separate them one from another, as a shepherd divideth his sheep from the goats:
33 And he shall set the sheep on his right hand, but the goats on the left.
34 Then shall the King say unto them on his right hand, Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world:
35 For I was an hungred, and ye gave me meat: I was thirsty, and ye gave me drink: I was a stranger, and ye took me in:
36 Naked, and ye clothed me: I was sick, and ye visited me: I was in prison, and ye came unto me.
37 Then shall the righteous answer him, saying, Lord, when saw we thee an hungred, and fed thee? or thirsty, and gave thee drink?
38 When saw we thee a stranger, and took thee in? or naked, and clothed thee?
39 Or when saw we thee sick, or in prison, and came unto thee?
40 And the King shall answer and say unto them, Verily I say unto you, Inasmuch as ye have done it unto one of the least of these my brethren,
ye have done it unto me. (Mat 25:31-40 KJV)
14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:
15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another

16 In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.
(Rom 2:14-16 KJV)