There are a couple of caveats buried in there. First is we share what we know, not what we do not know. If in conversation you are asked about some aspect of Christ for which you do not have an answer, you actually do, it is, “I do not know, but I will find out and get back to you.” John 9 is probably the greatest example of this in Scripture.
The idea here is not that this gives and excuse not to share; rather it is impetus to engage in the Word. If we do not, I am not clear on what it is we would be sharing.