The way Calvinism is described to me by Calvinists
and from my own reading, God's predestination appears completely
arbitrary (although the thinking of the Jansenists was even more extreme).
Not only would this make God out to be unjust, it also
contradicts so many other aspects of Christian teaching. There really would be
no need for anybody to 'opt for God' if salvation has been predetermined. This
also negates the need for evangelism.
The only way I can make all the Scriptures make sense
as a whole is to see what is said in Ephesians about 'predestination' in the
context of God acting in accordance with his foreknowledge. Everything then
interlocks and makes perfect sense.