There are no doubt aspects of God which we cannot grasp but when it comes to holiness, what is necessary and able to grasp is revealed in Jesus.
Trying to get a handle on holiness independent of Jesus as I see it, leaves the imagination to go skewed as is evident in 'religious' unveiling.
It’s certainly true we have an accurate picture of God in Christ...he tells us that very thing! But, I think we need to remember that when Christ came the first time, it was in humility and humanity. Which means that even though he was still divine, his divinity was not on full display. When we speak of Gods holiness, we think of Moses begging to look upon God, and God agreeing that Moses may look upon Gods train, and only when God covers Moses’ face with his own hand. And that left Moses’ face shining with such intensity that the Israelities begged Moses to cover his face with a bag!
If Christ had been displaying such glory and holiness as he walked among us we wouldn’t have survived. And also no one would have doubted who he was!
We get a glimpse of this nature on the Mt of transfiguration, but even then we must say his true divine nature was...muted. After all, Peter, James and John all survived the incident and did not come down with glowing faces.
My point is: while we can absolutely look at Christ to see the nature of Gods love, mercy and grace...many other attributes as well...there are other things that we must look elsewhere in scripture for a complete picture of. And I believe the holiness of God is something that only comes together when we consider both Old and New Testament revelation of who God is.