2 Pet 1:10

Per Calvinism, election cannot be changed by people; God elected people before they were born, and they themselves can not affect such election. 

However, Peter suggests differently, contradicting Calvinism's portrayal of election as being to salvation.

Wherefore the rather, brethren, give diligence to make your calling and election sure: ... - 2 Pet 1:10

This verse refutes election as being to salvation. For it tells people to try to make their own election sure.

How can anyone, per Calvinism, give diligence to make their calling and election sure? Aren't they already elect - or not elect - by God, with no ability in themselves to do anything that would affect or change that election? Isn't there nothing that anyone can do to make their own election sure?

One possible Calvinist response might be, "That does not mean for the individual to actually try to make their election sure. What it really means is for them, if they already are elect, to try to do what God ordained for them to do. Because God ordained for them to be told to try to do it." This is similar to the Calvinistic response to the question, Why evangelize if the elect will definitely be saved?, the response being, Because God ordained the method.

We then might ask Calvinism the following:

Why does God tell people to do something that He does not mean for them to do?

Why does God not tell people to do something that He does want them to do?

Why does God ordain for people to be told to do something they are going to do anyway - since it's ordained they would? Why does God tell people to try to do what they cannot do - make anything sure that God already ordained?

In short, this verse with Calvinism's concept of unconditional election leads to either apparent contradiction, or questions  and problems. The problems can be solved by rejecting the Calvinistic concept of unconditional election.