The perfect time to teach such an idea would have been in the parable of the soils regarding soil #2. Yet, Jesus said nothing about losing salvation. His only point was that such a person doesn't produce fruit.
11 “Now the parable is this: The seed is the word of God. 12 Those by the wayside are the ones who hear; then the devil comes and takes away the word out of their hearts, lest they should believe and be saved.13 But the ones on the rock are those who, when they hear, receive the word with joy; and these have no root, who believe for a while and in time of temptation fall away. 14 Now the ones that fell among thorns are those who, when they have heard, go out and are choked with cares, riches, and pleasures of life, and bring no fruit to maturity. 15 But the ones that fell on the good ground are those who, having heard the word with a noble and good heart, keep it and bear fruit with patience. Luke 11-15
So you believe people who fall away, and never produce fruit and saved?
Here is what Jesus said about those who produce no fruit.
Every branch in Me that does not bear fruit He takes away; and every branch that bears fruit He prunes, that it may bear more fruit.
John 15:2
If anyone does not abide in Me, he is cast out as a branch and is withered; and they gather them and throw them into the fire, and they are burned. John 15:6
Those who are "in Christ", that don't remain connected "in Christ", end up thrown into the fire and are burned.
As He said to His servants on the Day of Judgement:
41 “Then He will also say to those on the left hand, ‘Depart from Me, you cursed, into the everlasting fire prepared for the devil and his angels:42 for I was hungry and you gave Me no food; I was thirsty and you gave Me no drink; 43 I was a stranger and you did not take Me in, naked and you did not clothe Me, sick and in prison and you did not visit Me.’
44 “Then they also will answer Him, saying, ‘Lord, when did we see You hungry or thirsty or a stranger or naked or sick or in prison, and did not minister to You?’ 45 Then He will answer them, saying, ‘Assuredly, I say to you, inasmuch as you did not do it to one of the least of these, you did not do it to Me.’ 46 And these will go away into everlasting punishment, but the righteous into eternal life.”
Matthew 25:41-46
‘Depart from Me, you cursed, into the everlasting fire prepared for the devil and his angels:
The reason: for I was hungry and you gave Me no food; I was thirsty and you gave Me no drink; I was a stranger and you did not take Me in, naked and you did not clothe Me, sick and in prison and you did not visit Me.’
JLB