What a great verse to discuss! This would be one awesome discussion if we all stick to the topic! I think we should look at it with reference to the previous verses.. Jesus is talking about false prophets and how they could be deceiving.. And He follows that with this.. I don't think we should take this to say one can fall from grace..
Looking at the previous verses.. This is what it would mean.. We should not be deceived that signs and wonders authenticate or validate Christian faith.. It is validated only by fruits.. Just because a person is performing signs and wonders, we cannot go behind him and seek after signs.. I think it is talking about people being deceived into believing what Christian faith means.. There are many today who seek after signs.. And if signs are not there, then it is not genuine.. That is a dangerous thing.. They are living in this earth thinking they are saved.. But Jesus is saying, what they do really does not matter.. Because false signs could be done through evil force as well..