Josiah writes,

"John the Baptist himself doubted at times. Jesus described him as the greatest man who ever lived, and yet there he was in jail, and he sent Jesus a message asking "Are you the Messiah, or do we look for another?"Was John the Baptist saved at the time? Absolutely! But in a moment of weakness, doubts began to arise, and he needed a little extra reassurance from Jesus."

My response: First of all, John the Baptist was living before the Resurrection. So Christ has not yet proven that he was the Messiah (by being raised from the dead). Thus, John the Baptist believed the Messiah was to come, but he had doubts about who exactly the Messiah was (since the proof that Jesus was the Messiah had not yet been performed).

But a believer today has the infallible proof that Jesus is the Christ, so it's impossible for them to doubt.

And can someone believe that Jesus is the Christ, and doubt even for a moment they are a believer? Can a man be standing in the light of the sun and not realise he is in the light? Of course not. In the same way, its impossible to have the "light of Christ shining into your heart" 2 Cor 4:6 and doubt at any time you are in the light.

Here's my theory.

John the Baptist was living before Pentecost and lacked the New Covenant understanding that all believers have today. The ONE DECISIVE PROOF that Jesus was the Christ, was the Resurrection....

"This is an evil generation. It seeks a miraculous sign, and a sign will not be given to it, EXCEPT THE SIGN OF JONAH the prophet." Luke 11:29

John was living before the Resurrection and didn't have the proof.

However, you ask...

"Was not John witness to the Spirit of God descending like a dove upon Jesus, and did he not hear the voice from heaven saying "This is my beloved Son, in whom I am well pleased."?"

Yes, you're right. However, God had not proven this statement to be true, until he raised Jesus from the dead.

"God.. now strictly commands all men everywhere to repent ... having given PROOF TO ALL BY RAISING HIM FROM THE DEAD." Acts 17:31

This was exactly John the Baptist's problem too. These disciples and John couldn't work out exactly how Jesus was redeeming Israel, when it seemed nothing was happening.

Here is Christ's response:

"And He said to them, O foolish ones, and slow of heart to believe on all things which the prophets spoke! Was it not necessary for the Christ to suffer these things, and to enter into His glory? And beginning from Moses, and from all the prophets, HE EXPLAINED TO THEM THE THINGS ABOUT HIMSELF in all the Scriptures." (Luke 24)

Christ had to explain EXACTLY HOW He redeemed Israel because the disciples lacked that knowledge.

It was only AFTER THE RESURRECTION that the Holy Spirit revealed that Jesus was the Christ, by raising Him from the dead, and using the Apostles to explain the proof.

So the disciples and John the Baptist had DOUBTS ABOUT CHRIST BEFORE the Resurrection / Pentecost because they LACKED UNDERSTANDING of how Christ redeemed Israel. The Holy Spirit had NOT YET REVEALED this explanation. So when the disciples doubted that Jesus was the Christ, they were NOT rejecting the Holy Spirit's testimony (since it had not been revealed yet). Thus, Christ says...

"And whoever speaks a word AGAINST THE SON OF MAN, it SHALL BE FORGIVEN him."

The disciples had doubts that Jesus was the Christ, but they still believed the Christ was to come. In other words, they believed what the Holy Spirit had fully revealed -- that Jesus was the Christ -- but they had doubts about what the Holy Spirit had yet to reveal. And since the Holy Spirit had not yet proved Jesus to be the Christ (by raising Him from the dead), they were not blaspheming the Holy Spirit.

"And whoever speaks a word AGAINST THE SON OF MAN, it SHALL BE FORGIVEN him. But whoever speaks against the Holy Spirit, it shall not be forgiven him, not in this age nor in the coming one." Mt 12:32

As final example, consider Peter. He was a saved Jew who believed the Messiah was to come, and even confessed that "Jesus was the Christ". Yet, his faith had NOT BEEN CONFIRMED by the Resurrection. So Christ says Peter will be CONVERTED to the New Covenant understanding.

"But I have prayed for thee, that thy faith fail not: and when THOU ART CONVERTED [epistrepho], strengthen thy brethren." Luke 22:32

Conclusion

Believers today have the faith of the Apostles (2 Pet 1:1) and NOT the faith of the disciples. The disciples had doubts because they were living BEFORE the ONE DECISIVE PROOF, but the moment they UNDERSTOOD at Pentecost they NEVER doubted again. In fact, John says that anyone who denies Jesus is the Christ is an Anti-Christ.

Believers never doubt that the suffering and death of the Son of God is adequate to redeem and forgive them. They have God’s infallible promise that they are justified -- “Everyone believing in this One is justified from all things” (Acts 13:39). God also tells believers that they will sin, and that they are justified from all sin, so believers never doubt that God saves sinful people, including themselves. Therefore, God can and does convince justified people that they are actually justified, and there are no believers walking around ignorant of their justification. People who doubt they are justified are not justified. Believers have the infallible testimony of the Holy Spirit witnessing to the infallible Word, and this is always 100% sufficient to convince them that God will save ALL His people based on the atoning blood and imputed righteousness of Jesus Christ alone.