Quote 1
"He has all along, since his coming, been trying his power, slowly but surely; that big child-brain of his is working." (23.4)
Van Helsing assures the rest of the Crew of Light that even Dracula's brain is different from theirs—it's a "child-brain."
Quote 2
In his life, his living life, he go over the Turkey frontier and attack his enemy on his own ground; he be beaten back, but did he stay? No! He come again, and again, and again. (24.15)
Dracula has always been persistent. In the past, he's invaded other countries, and Van Helsing uses his knowledge of history to help predict what Dracula will do now.
Quote 3
Thus are we ministers of God's own wish: that the world, and men for whom His Son die, will not be given over to monsters, whose very existence would defame Him. He have allowed us to redeem one soul already, and we go out as the old knights of the Cross to redeem more. Like them we shall travel towards the sunrise; and like them, if we fall, we fall in good cause. (24.12)
Because Romania borders on Turkey, Van Helsing compares their mission with the old Crusaders, or the "knights of the Cross," who traveled east ("towards the sunrise") to Jerusalem to "reclaim" it for Christians. Van Helsing believes that they're on a mission from God.