Second, it may be helpful to take the opportunity to do a little bit of background work on metaphor so that you remind yourself, and if preaching your congregation, just how important a clear understanding of metaphor is for interpreting the Bible. You do not need to be a person with heavy interest in literary matters to renew your understanding of metaphor. Understanding metaphor is important in almost everything we do from relating to our closest friends and partners to determining the purposes and goals in our lives. You can use I. A. Richards helpful descriptive language of tenor and vehicle to describe metaphor or merely the general notion that metaphor deals with similarities of two things/ideas/concepts/objects. Just list some of the metaphors used in the Bible to speak of God, Jesus, Holy Spirit, or church and you will put yourself on a path to better understanding the many biblical texts that use metaphor.
Third, one of the keys to understanding 1 Corinthians 3:2 is to read carefully that Paul is speaking in the first person, "I." He is speaking as the mother. His letter is really about caring for the way the Corinthian house churches have gotten off message with their internal bickering just to mention one matter. Paul is not talking generally about feeding or teaching or giving care. The first person is important because if you look at the many texts in the Hellenistic world that use milk and solid food (even take a look at Hebrews 5:12-14) as contrasting types of food you may miss Paul's point. Paul is not contrasting milk and solid food. He is contrasting the diet of the Corinthians with the food/message he has offered, namely Jesus Christ crucified. Paul tells the Corinthian house churches I am trying to care for you so he picks the mothering language throughout. We should not be surprised because much of the context of the letter uses family imagery.
Paul does use paternal metaphors but the maternal metaphors come to the front when he is dealing with needs that extend over a period of time.
Finally, Gaventa has brought to our attention as others have done that the association of milk and Paul occur repeatedly as a positive image in post biblical traditions. The scene in the second century document called the Acts of Paul is one of the most vivid. Seemingly at Paul's execution it was not blood that spurted from his having had his head chopped off. It was milk! There is little way that the force of our 1 Corinthians passage could have any negative connotations about Paul offering milk to he Corinthians as opposed to solid food. What Paul offered was milk and solid food but always the single diet of the cross which is the saving power for all.
**The image is from the cover of the Gaventa book which depicted Paul by the artist, Jay Mulford.