Where does Paul mention that Jesus was executed by the Romans as a criminal, let alone tried and convicted by Pontius Pilate? Nowhere. Paul gives no historical context to the crucifixion. The closest that he comes to hinting at who crucified Jesus is in 1 Corinthians 2:6-8:

Yet among the mature we do speak wisdom, though it is not a wisdom of this age or of the rulers of this age, who are doomed to perish. 7But we speak God’s wisdom, secret and hidden, which God decreed before the ages for our glory. 8 None of the rulers of this age understood this; for if they had, they would not have crucified the Lord of glory.

The phrase translated in English as “rulers of this age” could refer to secular authorities. But it could also refer to spiritual forces that have power and authority over Earth. Thus, the passage is ambiguous. Paul, however, gives hints elsewhere of what he might have meant:

Ephesians 6:12
For our struggle is not against enemies of blood and flesh, but against the rulers, against the authorities, against the cosmic powers of this present darkness, against the spiritual forces of evil in the heavenly places.

Here Paul explicitly contrasts the “rulers” and “authorities” with flesh and blood people, and equates them with “cosmic powers” in “heavenly places.”

Now, let’s take a look at what else Paul said about the crucifixion:

1 Corinthians 1:22-24
22 For Jews demand signs and Greeks desire wisdom, 23but we proclaim Christ crucified, a stumbling-block to Jews and foolishness to Gentiles, 24but to those who are the called, both Jews and Greeks, Christ the power of God and the wisdom of God.

Curiously, Paul proclaims Christ crucified. Why does he have to proclaim this if it was a recent historical fact? Couldn’t he just say that “we give testimony to Christ’s crucifixion by the hands of the Romans on Calvary?” Well, remember from where Paul has received his gospel. Paul did not receive it from men. He did not hear testimony from any disciples of Jesus claiming to have been there and witnessed everything with their own eyes. Instead, he had a vision of the risen Jesus, like the other apostles he mentions in In 1 Corinthians 15. In that same chapter, Paul also indicates that Jesus’ death was “in accordance with the scriptures.” So, his other source of information is the Jewish scriptures.

Paul is proclaiming the “hidden” message of God. I speak of God’s secret wisdom, a mystery that has been hidden and predestined by God for our glory before time began.

So where did this crucifixion take place? Paul doesn’t say. Could it have been a terrestrial crucifixion of a real man? The gospels transformed the message into just such a story, but Paul doesn’t speak of it unambiguously or explicitly that way. Could it have been a spiritual crucifixion that took place in the Heavens against the will of the cosmic powers? Possibly. At least, it seems to be consistent with the language that Paul uses throughout this letters.

But why a crucifixion of all forms of execution?

Certainly, crucifixion was a common form of execution at the time, at least in the Roman empire. If Christ was to die, whether symbolically or literally, that’s one way to do it. To complicate the matters, Paul several times also speaks symbolically about his own “crucifixion”:

Romans 6.6:
We know that our old self was crucified with him so that the body of sin might be destroyed, and we might no longer be enslaved to sin.

Galatians 2.19:
For through the law I died to the law, so that I might live to God. I have been crucified with Christ;

Maybe it is an ad hoc rationalization, but I think that Paul and some of the early Christians associated with him sincerely believed that the message of Jesus’ death and crucifixion was encoded within the Jewish Scriptures and that it was their job as apostles to reveal the hidden language. The gospel writers took this a step further by constructing events in Jesus’ “life” based on passages from the Jewish Scriptures.