Donovan wasn't too smart
Donovan clearly believes in God, Jesus Christ and the Grail's power to give immortality. Yet he does evil things including murder to obtain the Grail for himself and he thinks God's going to let him have it and live forever.
A prophet of God even warns him that the Grail will give him nothing but everlasting damnation. He looks at the dying prophet with contempt and ignores him. He even took part in the mass-murder of the prophet and his men.
And he's working with the Nazis. I mean use your God-given brains Donovan.
Since he's a Christian believer why doesn't he just live a virtuous Christian life and get eternal life that way?
Although Henry says Elsa didn't believe in the Grail's power but just saw it as a relic Elsa herself told Indy that she believes in the Grail and not the Swastika.
And then you have the French archaeologist in Raiders of the Lost Ark who believes in God and yet he's a thief and a murderer and he thinks God is going to be his good friend if he finds the Ark. I would think that he as an archaeologist and a believer in God would have read the Bible and know what it says happened to evil men who open the Ark.
These have got to be some of the stupidest movie villains ever.