In Matthew 13, Jesus taught this parable: “A sower went out to sow. And as he sowed, some seeds fell along the path, and the birds devoured them. Other seeds fell on rocky ground and immediately they sprang up. But the sun scorched them and they withered since they had no root. Other seeds fell among thorns and the thorns grew up and choked them.
“But other seeds fell on good soil and produced grain, some a hundredfold, some sixty, some thirty.”
The disciples asked Him the meaning of the parable, so Jesus said, “When anyone hears the word of the kingdom and does not understand it, the evil one snatches away what was sown in his heart. This is what was sown along the path.
“As for what was sown on rocky ground, this is the one who hears the word and immediately receives it with joy, yet he has no root in himself. He endures for a while, and when tribulation or persecution arises on account of the word, he falls away.”
“As for what was sown among thorns, this is the one who hears the word, but the cares of the world and the deceitfulness of riches choke the word and it proves unfruitful. As for what was sown on good soil, this is the one who hears the word and understands it. He indeed bears fruit,” because his faith is genuine.
There are people who might appear to be Christians for a time. But when their faith is no longer convenient, it turns out to be a passing opinion. They were never truly saved.
1 John 2:19 says, “They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, so it might become plain” they were never part of Christ’s body, when we understand the text.