Your formal cause is what you are. This seems very simple but has been the greatest cause of division in the Church since Christ's time on earth. At the Last Supper, Jesus offered bread and body, saying that it is His body and blood. He did not say these look like body and blood, or represent body and blood; He said they are body and blood. That is what they are. On that very occasion, some of Jesus' followers couldn't handle that particular form(al cause) of the bread and wine, and left.
And what is our formal cause? Is it enough to say we're human beings? Does that tell the whole story?
What a piece of work is a man, how noble in reason, how infinite in faculties, in
form and moving how express and admirable, in action how like an angel, in
apprehension how like a god! the beauty of the world, the paragon of animals—and
yet, to me, what is this quintessence of dust? - Hamlet
Hamlet calls us a quintessence of dust - the embodiment of dust or, you might say, dust made flesh. Think back to your last Ash Wednesday and his comment takes on a new, Christian meaning.
Dust is certainly not beautiful or a paragon, compared to the myriad animals that God has made. What raises us up to that level? What allows our mortal faculties to one day become infinite, for our knowledge to become God-like? Only one can - God.
On a biological level, we're not fundamentally different from other life on earth. Something in this pile of dust other than biology raises us up; something we can't see changes us. Just like the form of that bread and wine changed at Christ's will, so does our form change. We stop being animals and start being, like the bread and wine, the Body and Blood of Christ.
We are created from dust to become part of God's Body. Our form - our formal cause - is meant to be Christ.
Hamlet quoted from: http://www.enotes.com/shakespeare-quotes/what-piece-work-man