Did you get this idea from Colossians? I can see how someone might understand Paul to be talking about "zeal" in the Colossians passage if by "zeal" we mean "readiness" and "eagerness." As we can see from the context, Paul wants his readers to pray that God would open a door for the word. And so I agree with you to a point.
Originally Posted by JohnDB
Notice, however, Paul prays for two things: 1) that God will open a door to the gospel, and 2) that Paul might be clear in his teaching. So, now that I look at this more carefully, it would appear that Jesus and Paul are, indeed, using the metaphor of salt in a similar way.
Devote yourselves to prayer, keeping alert in it with an attitude of thanksgiving; praying at the same time for us as well, that God will open up to us a door for the word, so that we may speak forth the mystery of Christ, for which I have also been imprisoned; that I may make it clear in the way I ought to speak. Conduct yourselves with wisdom toward outsiders, making the most of the opportunity. Let your speech always be with grace, seasoned with salt, so that you will know how you should respond to each person. Colossians 4:2-6
I always thought that the phrase "seasoned with salt" was synonymous with "grace" in the sense that just as salt makes food more palatable, consideration for others will make the truth easier for others to hear.
It's also possible, that the phrase "seasoned with salt" is a metaphor for "distinctiveness", of which "clarity" is also a related idea. So, it's possible that Paul is saying two things: 1) speak to others with a disposition to kindness and compassion, while at the same time 2) seek to be clear and straightforward with the Gospel.
If I am right about this, then Jesus and Paul are using "salt" as a metaphor for clarity and a lack of compromise and evasion.