What immortal hand or eye, Could frame thy fearful symmetry? (lines 3-4)
We can’t really think of a time when a poet would use "immortal" without implying it might be about God or a god. These two lines are very versatile when it comes to interpretation, but the idea of something immortal having a hand or eye like a man, as well as some sort of agency (the ability to do things), should immediately ring the Christian God bell if the poem is written in English – there’s just no way around the big guy.
In what distant deeps or skies. Burnt the fire of thine eyes? (lines 5-6)
The first line of this excerpt is really the important one for this theme. A reference to a mysterious, distant deep (something underground/below us) brings to mind the traditional idea of Hell as someplace you might get to if you dug far enough. As for a mysterious, distant sky, that might refer to the opposite: Heaven. Especially within the context of the Tyger being created in one of these places, the supernatural factor goes way up.
Did he who made the Lamb make thee? (line 20)
You’ve probably heard of "the Lamb of God," the lamb being Jesus Christ, God’s son who was the sacrifice to end all sacrifices for the sins of mankind. So the quote above basically asks, "Did the guy who made Jesus Christ make you, Mr. Tyger?" Coming from the tradition that says God made Jesus Christ, the "maker" of Jesus was God. So, "Did God make you, Mr. Tyger?"