What is the ruling on reading novels that include phrases specific to Christians, such as: "Say your last prayers" and "Thank God"?

There is no objection to either phrase. The questioner’s objection to them is because they are frequently used by Christians or attributed to them. Therefore, there is no harm in reading novels that include them, so long as they do not contain other objectionable material. However, it is preferable to avoid them and occupy one's time with what is more beneficial.