Calvinists believe in the doctrine of the Perseverance of the Saints (Eternal Security or Once-Saved-Always-Saved), which says that those whom God has called can never fall away. Because God chose some to be saved, he keeps them from apostasy and those who do apostatize were never truly born again.
Arminians believe that salvation is secure in Christ and protected from all external forces, but it is conditional on us remaining in Christ and it can be lost through apostasy. Salvation is conditioned on faith, therefore perseverance is also conditional. Apostasy (turning from Christ) can be committed through a deliberate, willful rejection of Jesus and renouncement of previous faith.
Can you imagine the following conversation taking place with a sane person?
A: Did you hear? John fell off the roof.
B: Oh dear, Was he hurt?
A: Not really – he never was truly on the roof.
B: But I thought you said he fell off the roof?
A: Yes, but the fact that he fell off the roof proves that he was never really on the roof in the first place.
If this line of reasoning sounds stupid to you, chances are you’re an Arminian.