Considered a man of mild temperament, Arminius was forced into controversy against his own choice. He had earlier affirmed the Calvinist view of predestination, which held that those elected for salvation were chosen prior to Adam's fall, but he gradually came to have doubts. To him predestination seemed too harsha position because it did not allow human decision a role in the achieving of salvation. Hence Arminius came to assert a conditional election, according to which God elects to life those who will respond in faith to the divine offer of salvation. In so doing, he meant to place greater emphasis on God's mercy
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From my favorite encyclopaedia
What he is saying is God can take away your salvation if you sin too much. Christians would have none of this and branded it heresy because of the doctrine of predestination.
I can define predestination for you if you want me to, but the main thing is to accept predestination as the correct doctrine and Arminianism as the false one since that will prevent you from being excommunicated.
Here's a verse on predestination:
For those whom he [God] foreknew he also predestined to be conformed to the image of his Son, in order that he might be the first-born among many brethren. And those whom he predestined he also called; and those whom he called he also justified; and those whom he justified he also glorified (Rom. 8:29–30).
That means if God meant for you to be a Christian, you have NO "free will" to back out of it. You are basically "stuck" willfully being God's slave. That means if he truly is a Christian, he can't possibly lose his salvation. But it's not as bad as it sounds...
P.S. There is the one DEADLY SIN which even Christ cannot forgive. If a Christian commits it, there will be no forgiveness whatsoever. Since you don't know what is, you can't commit it, I shan't tell you. For your own sake.