Can a Christian sin after he has been saved? Here is what the Bible says:
No one born of God makes a practice of sinning, for God’s seed abides in him, and he cannot keep on sinning because he has been born of God.
1 John 3:9
It doesn’t say a Christian will never sin but that he can’t keep on living a life of sin. Someone who professes to be a Christian but lives a life of sin proves that his profession is false. A comparison of two kings, David and Amaziah, shows the difference.
In the parable of the sower part of the seed fell on rocky ground.
Other seeds fell on rocky ground, where they did not have much soil, and immediately they sprang up, since they had no depth of soil, but when the sun rose they were scorched. And since they had no root, they withered away.
Jesus explained the meaning to his disciples.
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, but endures for a while, and when tribulation or persecution arises on account of the word, immediately he falls away.
Judah had a king, Amaziah, who was an example of this kind of seed.