This is a critique of a particular type of person who identifies as being Christian. There are many Christians who embody the beliefs of Christ and who demonstrate the Biblical "fruits of the spirit": love, joy, peace, patience, kindness, goodness, gentleness, faithfulness, and self-control.
And then there are others like Manley. By hiding condoms (along with whiskey and obscene playing cards) in a fake Bible, he proves to be a hypocrite, hiding behind the façade of Christianity to gain trust in certain circles and then exploit that trust. There is nothing Christlike about Manley, just as there is no word of God in that hollowed-out Bible.
O'Connor is seemingly differentiating between truly "good" people, whether they identify as Christian or not, and those who merely curate an outer Christian image. Much like the outer appearance of this Bible, these pseudo-Christians are fakes who fail to represent Christ or the truths of the Christian faith. Their purposes are self-serving, as are the items which Manley carries in his fake Bible.
Manley's efforts to use the Bible to hide darker sins reflects a difficult truth about those who claim to be Christian. The true assessment of a person's faith must therefore consider his inner heart.