"I am the true Vine and my Father is the Vinedresser. Every branch of mine that does not bear fruit, he takes away, and every branch that does bear fruit he cleanses (kathairo). You are already made clean (katharos) through the word I spoke to you. Remain in me and I in you. As the branch cannot bear fruit by itself, unless it remains in the vine, neither can you unless you remain in the Vine. I am the Vine and you are the branches. He who remains in me and I in him is he that bears much fruit for apart from me you can do nothing. If a man does not remain in me, he is cast forth as a branch and withers and the branches are gathered and thrown into the fire" (John 15:1-6)
This passage is pretty plain. Christ is the Vine, the Father is the Vinedresser and the disciples are the Vine's branches and receive their life through the Vine. Christ the Vine is exhorting his disciples the branches that if they do not remain in him by bearing fruit to God they will be cast out as an unproductive branch and be thrown into the fire. The words kathairo and katharos are used here to show the disciples have been cleansed just as the pruning of an unproductive branch cleanses the whole vine.
One can only remain in Christ if one is first in Christ and the situation of being cut off from the Vine would be quite useless to teach if it were not within the realm of possibility. It is rather obvious that it is quite possible for the disciples not to remain in Christ and be cut off which is why he says, "If a man does not remain in me...." There would be absolutely no point in Jesus making such a statement unless of course being cut off from Christ was indeed a possible scenario. Nor does Christ say, "Since you will remain in me..." He rather exhorts them to remain in him, something which is rather pointless if it were impossible to not remain in him and hence be purged from the Vine and thrown into the fire.