By this I do not mean what is usually meant in grammar class, using two different and unrelated metaphors in the same phrase of an analogy. Like this one: "That's awfully thin gruel for the right wing to hang their hats on." (MSNBC, September 3, 2009). Rather, what we refer to here is assuming that every Biblical metaphor means the same thing in every context. Prime example:
For though by this time you ought to be teachers, you need someone to teach you again the basic principles of the oracles of God. You need milk, not solid food, for everyone who lives on milk is unskilled in the word of righteousness, since he is a child. But solid food is for the mature, for those who have their powers of discernment trained by constant practice to distinguish good from evil. (Heb 5:12-14)
So put away all malice and all deceit and hypocrisy and envy and all slander. Like newborn infants, long for the pure spiritual milk, that by it you may grow up into salvation-- (1Pet 2:1-2)
I don't know how many times I have heard these two passages equated just because babies and milk are mentioned in both. Here is the problem: they speak of two entirely different issues and if you don't separate them, you miss half the teaching.
In the Hebrews passage, the writer rebukes those who have not progressed in their knowledge of the word. They are still babies who need milk. They ought to have matured into adults who can handle a T-bone steak, but they cannot. They have not "trained their powers of discernment." In this case, the milk is the first principles, the ABCs of being a Christian, the easy stuff, and being a baby is something you definitely do not want to be.
Peter, on the other hand, says we should desire the spiritual milk in the same way an infant desires its mother's milk. The "baby" in this passage is a good example, not a bad one. We have all seen a hungry infant open its mouth and grunt for that milk over and over until it is fed. All of us are supposed to be like that little baby, hungering for the spiritual milk, no matter how long we have been Christians. In this case, the metaphor is about hunger, not just for the Word but for all spirituality, and the baby is something you want to be.
These two passages may use some of the same words, but they are not about the same thing. All it takes is a little slow reading of the entire context, and then maybe a little thought—pretty obvious thought as a matter of fact. Perhaps one could even call it "milk."
Here's another set:
According to the grace of God given to me, like a skilled master builder I laid a foundation, and someone else is building upon it. Let each one take care how he builds upon it. For no one can lay a foundation other than that which is laid, which is Jesus Christ. Now if anyone builds on the foundation with gold, silver, precious stones, wood, hay, straw— each one's work will become manifest, for the Day will disclose it, because it will be revealed by fire, and the fire will test what sort of work each one has done. (1Cor 3:10-13)
Now in a great house there are not only vessels of gold and silver but also of wood and clay, some for honorable use, some for dishonorable. Therefore, if anyone cleanses himself from what is dishonorable, he will be a vessel for honorable use, set apart as holy, useful to the master of the house, ready for every good work. (2Tim 2:20-21)
This one is a little trickier. Both passages mention gold, silver and wood, along with a few other materials. However, in the Corinthian passage, Paul uses the various building materials to say that all of us are different. We each use whatever our abilities are to build on the spiritual foundation. Some of us have greater abilities than others, but God will be the judge of how we use those abilities. Being a "wood" disciple is not necessarily bad as long as we are doing what we can with that wood. The analogy here is our various abilities, a subject he will eventually come back to in chapter 12, using the body as his analogy.
However, in the Timothy passage, we are talking about pots, and what the pot is made of determines whether it is honorable or dishonorable. He goes on to say that we will be honorable vessels, i.e., gold or silver, if we cleanse ourselves, making ourselves suitable for God's use. We will be dishonorable, wood or clay, if we don't. Do you see the difference in how this metaphor is used? In the first, wood is not necessarily wrong, but in the second it is definitely wrong.
These are not the only two by a long shot. In one place Christ is the foundation; in another the apostles are. Stop tying yourselves in knots and just realize that you are using figurative, not literal, language—metaphors, in this case. The way some people go on, you would think Jesus and the apostles were real stones! Separate your metaphors rather than mixing them and you will come much closer to the truths they are trying to teach you, without missing any of them.