Paul reported that a being of light spoke to him on the road to Damascus. The being claimed to be Jesus, but it was really Satan.
Our first clue as the being's true identity is Acts 9:7:
And the men that journeyed with him [Paul] stood speechless, hearing a voice, but seeing no one.
This visitation is starkly different from Jesus' actual appearances on earth after His death. For example, John 20:27:
Then said he to Thomas, "Reach out your finger, and behold my hands; and reach out your hand, and put it into my side: and be not faithless, but believing."
When He appears to His disciples, those that knew Him in life, Jesus has a body. But when a being of light appears to Paul (then called Saul), it does not reveal itself bodily, as Jesus would have.
Our second clue is what happens to Saul. He is blinded. Does that sound like something Jesus would do? Jesus returned sight to the blind.
So Jesus had compassion on [the blind men], and touched their eyes: and immediately their eyes received sight, and they followed him. (Matthew 20:34)
Jesus' light lets one see. But the light of the being claiming to be Jesus blinded Saul. We know that the Devil has the power to assume a pleasing shape, and that he was originally an angel of Light (thus his name: Lucifer).
Finally, consider who Saul was: a persecutor of Christians. He had already been serving Satan's purposes before the visitation, and he continued to do so afterwards. Satan wouldn't have been able to pull this trick on a disciple, someone who had actually met Jesus in life.
Satan's plan becomes pretty clear. First, he had sent his minions to try to destroy the young church. He sent people like Saul to imprison and kill Christians. But soon it became obvious that the Word was out, and not even Satan could stop it. So Satan changed tactics. If he couldn't destroy the church, he could infiltrate it and pervert it.
The history of the various Christian churches demonstrates how well Satan has infiltrated them. Bloody wars, horrible tortures, gross immorality, tremendous hypocrisy, greedy power-grabbing, and tribalism in general have marked many Christian churches throughout history.
The true apostles demonstrated the power of the Holy Spirit at Pentecost when they spoke in tongues. Whatever they said was heard by each hearer in the hearer's own language. Satan, unable to replicate this miracle, used Paul to confuse Christians about it. Paul popularized the idea that speaking in tongues means babbling incomprehensibly (the opposite of its true meaning). Even today you can see shady televangelists babbling as a way to impress (and dupe) their audiences.
Satan started his infiltration of the church by turning Saul into a false convert and sending him out among the gentiles. Many early churches were founded, guided, or influenced by Paul. Church leaders even canonized his epistles into the Bible, and they were especially exalted by the heretic Marcion. He is one of the Catholic church's favorite saints.
And all this even though Jesus Himself never prophesied of Paul's arrival.
Text of the Bible courtesy of Project Gutenberg.