Here's a prime example, Spurgeon says this -
Nay, the doctrine of justification itself, as preached by an Arminian, is nothing but the doctrine of salvation by works, lifted up; for he always thinks faith is a work of the creature and a condition of his acceptance. It is as false to say that man is saved by faith as a work, as that he is saved by the deeds of the law. We are saved by faith as the gift of God, and as the first token of his eternal favor to us; but it is not faith as our work that saves, otherwise we are saved by works, and not by grace at all.Of course, all Bible-believing Christians would certainly oppose such a view, loudly saying that one cannot be saved by works, so thus faith cannot be a work. Yet, Calvinists (which include me) like to point out that synergism makes faith a "work." Luther said the same thing in his Bondage of the Will as well. It's spoken of a lot.
Let me try to explain what I would mean by the statement. To head something off, he wasn't saying that all Arminians were damned. He outright rejects that idea elsewhere. I would say the same thing, though I would affirm with him that Arminianism is wrong, and dangerous doctrine.
But let me try to explain the "works" idea a little more.
The biblical position of justification is man is saved by God's free grace alone. We are saved by Christ's works (his righteousness), plus nothing of ourselves. We are not saved by obedience to the Law, nor by anything else that we try to do, but rather we are saved by Christ's righteousness (as he walked the earth), alone.
So why would I agree with Spurgeon when he says that "Nay, the doctrine of justification itself, as preached by an Arminian, is nothing but the doctrine of salvation by works"? Because if we honestly somehow are choosing God of our free will, then "accepting faith" is something wholly of man and not a gift of God. Some accept, some deny. Why do they accept it? Their free will.
What does this make "faith" (or "accepting faith")? It makes it something that God responds to rather than something God initiates, and ultimately salvation is finally dependent not upon the works of Jesus Christ, but my decision to accept that gift.
One might argue that the decision to accept the gift plays such a small part in the role of salvation that it's not really a "work." But does it really play such a small part? It's certainly a determining factor. Christ's righteousness the determining factor alone (Christ's works alone), but it is my acceptance of Christ's righteousness + Christ's righteousness. Both are necessary for salvation. This makes our justification a mixture of Christ's works and our work. Or, in short, essentially justification by works (our works).
Earlier, Phil Johnson posted a 8 part series on "why I'm a Calvinist, and . . . and why every Christian is a Calvinist of sorts." I would highly recommend it. It makes this point, elaborates it, and points out that actually all Christians are just inconsistent Calvinists.
Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, Part 7, Conclusion