Could it be He is saying, that if you only have a little of the truth it will be taken away?
In Romans 1, Paul makes some statements which may help you get an answer to this question.
He states that everyone 'knew God' (v 21) but they didn't all accept and apply themselves to 'the truth' (v 18). Certain consequences of this attitude followed - and will follow anyone who holds those attitudes, even today, even Christians.
Regarding your thought that Christians might have their measure of truth taken away from them, Jesus sets the terms of this in Matt 7:
20 Wherefore by their fruits ye shall know them. 21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? 23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
24 Therefore whosoever heareth these sayings of mine, and doeth them, I will liken him unto a wise man, which built his house upon a rock:
All the time Jesus was doing miracles (healing and casting out demons), it was to show forth the mighty works of God. James in his epistle sets out the same challenge - faith + works - and Peter does when he exhorts to hear the word and do it.
We can hardly imagine Mary listening at Jesus' feet yet NOT sharing what she'd heard with others, just as many who had been healed or delivered shared their wonderful news.
The gospel can be shared in many different ways, including across language barriers, without words, so only God could make that call - to condemn them for things they ought to have said (or done).
29 For unto every one that hath shall be given, and he shall have abundance: but from him that hath not shall be taken away even that which he hath.
This raises a question: what is a 'talent', in this parable?
