For proof that Christians can lose their connection with Jesus and be lost, read John 15:1-6. There Christ explains one of the great mysteries of eternal life. "I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing. If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them and cast them into the fire, and they are burned." Verses 5, 6.
The secret of continued life is continued abiding. If a person does not abide in Christ, he withers, dies, and is finally burned up. This proves that the relation of a believer to Christ is never a static thing based only on a past experience. It is a current, mutual sharing of a common life which is drawn from Him "who is our life" (Colossians 3:4). When the branch is separated from the vine, the source of life is gone, and only death can result. These words of Jesus are too clear to be misconstrued. Even believing, trusting Christians who are connected to the living vine may choose to separate from the vine. When they do, they die and will be cast into the fire and burned. Nothing can wither and die that has not previously been alive.
Jesus taught the same solemn truth about losing eternal life in the parable of the sower. In explaining the seed which fell among the thorns and the rocks, Jesus said, "Those by the way side are they that hear; then cometh the devil, and taketh away the word out of their hearts, lest they should believe and be saved. They on the rock are they, which, when they hear, receive the word with joy; and these have no root, which for a while believe, and in time of temptation fall away." Luke 8:12-13.
There are several things to note about this parable. First, only one class will finally be saved - the ones who brought forth much fruit. The groups represented by the wayside and rocks will not be saved. In verse 12, the wayside hearers did not have a chance to "believe and be saved," but in the next verse the stony ground hearers do "for a while believe." What kind of "believing" is this? According to verse 12, it is the kind that saves. So the ones who believed for a while were saved for a while, but in time of temptation they fell away. Eventually, of course, they were lost along with all the others, except the fruit-bearers. Here is an unequivocal teaching of our Lord that people can have a saving faith for a while, and yet lose it and be lost.