This statement is a result of the ecumenical councils of the Church. The dual nature of Jesus Christ with his death by crucifixion and resurrection is the detail of salvation for all who turn to the Holy Trinity. If Jesus were only human, there could be no salvation; other humans have been crucified and some have been innocent of the crime for which they were executed with no effect on anyone else. If they were mixed in some way, this would mean that something neither human nor Divine was crucified. It is only because the two are side by side (so to speak)in full cooperation with each other that He becomes a bridge between the human and the Divine.
The human nature of Jesus means that the crucifixion was an historic event. The Divine nature makes the history transcend time and space and is now available for all, past, present and future.
I would suggest you check out the articles on the seven ecumenical councils. I'm not sure if listing an URL here would be accepted by the modulators. So I'll wait a word from them on this matter.