What are Grendel's thoughts about God or gods throughout the epic?
Very little is stated about how Grendel, from the Epic Poem Beowulf, feels about God. While little is said, enough is given to the reader to establish that Grendel hated God.
Typically, in Anglo-Saxon literature, the antagonist of the hero (in this case Grendel) was characterized as a "God-Hater." Written from a Christian perspective, Beowulf is no different.
In the early description of Grendel it is stated that he is a descendant of Cain:
Grendel this monster grim was called,
march-riever mighty, in moorland living,
in fen and fastness; fief of the giants
the hapless wight a while had kept
since the Creator his exile doomed.
On kin of Cain was the killing avenged
by sovran God for slaughtered Abel.
Here, the reader is shown that Grendel has been exiled by God based upon the fact that he is a descendant of Cain. (What this refers to is the fact that Cain, jealous of God's acceptance of Abel's sacrifice, murders his brother. God sees the hatred in Cain and exiles him to darkness.) Therefore, given all creatures
Etins and elves and evil-spirits,
as well as the giants that warred with God
should be exiled to darkness as well. Given that Grendel is an evil spirit, he is exiled to darkness as well.
Grendel, angered that he cannot exist in light (and therefore in God's graces) wages war on the people of Heorot. Grendel, unable to enact his anger against God himself, wages war on the Heorot and its inhabitants (given they are able to exist in light).
Basically, Grendel despises God and, therefore, Christianity.