The few metaphors included in the Gospels have been extended to a sustained metaphor which assume the purpose of an allegory. In the Gospel of St.John:
I am the real vine and my Father is the gardener. He takes off every branch in me that does not bear fruit. He cleans every branch that does bear fruit, so that it will bear more (Jn. 5:12).
The message behind this allegorical passage is that all received grace only through Jesus. His Father fosters this life of grace. Every christian who sins loses sanctifying grace. But christians who live holy lives are perfected so that they grow in sanctity.
The Gospels comprise of a parable which approaches the form of allegory; for instance, the parable of the sower.
There was a man who went out to sow. As he scattered the seed in the field some of it fell along the path and the birds came and ate it up. Other seeds fell on the rocky ground, where there was a little soil … Some of the seeds fell along thorns which grew up and chocked the plants … But some fell in good soil, and the plants sprouted, grew and bore grain … (Mk. 4:3-8).
Mark uses the allegorical form to inform us the duties of his Apostles. Apostles are sent by God to preach when they preach God’s message, their words are heard by some people, but only as it were in passing. The devil does not give them a chance to listen to it properly. Other people listen, but their faith has no roots. In others it is materialism that chokes faith. In few others, faith produces sanctity.