The Character of the Expressions Used is Above All to Have Weight
34 The chief thing to be inquired into, therefore, in regard to any expression that we are trying to understand is, whether it is literal or figurative. For when it is ascertained to be figurative, it is easy, by an application of the laws of things which we discussed in the first book, to turn it in every way until we arrive at a true interpretation, especially when we bring to our aid experience strengthened by the exercise of piety. Now we find out whether an expression is literal or figurative by attending to the considerations indicated above.
Source: Christian Doctrine (New Advent)