There is a great passage in Paul's first letter to the Corinthians which may, I believe, shed light on the passage you quoted. In Chapter 2 of 1 Corinthians, the apostle Paul recalled how he presented himself to the Corinthians when he first came to them.
He did not come on strong, as would some eloquent orators, but he came on weak, so that the fruit of his preaching would be attributable only to God and not to him. Furthermore, his message of the cross was not framed in a polished style and accompanied by a dynamic delivery, with tightly-knit and persuasive arguments and infallible reasoning to buttress his message. No.
Worldly wisdom, of which Paul had spoken earlier in his letter (see Chapter 1:18 ff.) was not the vehicle of his preaching; rather, it was "in demonstration of the Spirit and of power" (2:4).
Then Paul goes on to say (and the bold print is mine),
Howbeit we speak wisdom among them that are perfect: yet not the wisdom of this world, nor of the princes of this world, that come to nought:
But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory: Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory. But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him. But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God (vss.7-10 KJV).
In looking back on the circumstances surrounding the crucifixion of his Lord, Paul realized, as did his Lord while hanging on the cross, that the Roman and Jewish authorities involved in the crucifixion (or as Paul called them, "the princes of this world") would not have crucified Jesus had they recognized who Jesus really was and had believed in him.
Unbelief takes many forms, is manifested in many ways, and afflicts all people--even Christ followers. The commonality in disbelief is distorted thinking about the truth, even when, as in Jesus' day, Incarnate Truth and "the powers that be" (both Jews and Romans) were face to face.
Call it what you will: spiritual blindness, unbelief fueled by hatred, lack of understanding, or even willful ignorance (i.e., not knowing either what one is doing or what is being done); Jesus realized that his captors either could not or would not understand the gravity of what they were doing to him.
That is why, I suggest, Jesus asked his Father to forgive his torturers, for although they knew at least intellectually what they were doing--namely, killing a political gadfly and a Jewish blasphemer, in their heart of hearts they did not really know, for had they known, they would not have crucified their Lord and Savior.