God holds a Christian in the palm of His hand
A good way to look at the assurance passages is to consider yourself to be held in the palm of God's hand. He will never drop you or make any mistake with you. Nor will He ever forsake you or reject you merely because you have let Him down. He knows perfectly well that you will let Him down and that you will inevitably sin. He is totally realistic about you. So, the real question is not whether God will drop us, but whether we will choose to throw ourselves out of His hand.
No person will ever be cut off or lose their salvation solely because they let God down or made mistakes or failed Him. If it was based on that, we would all have to be cut off and rejected every single day, because that is how often we fail God or let Him down, at least in my case.
It is not, therefore, based on that. It works the other way round. As we have seen with the 'fruit' passages above, the real question is not whether you have let God down. It is whether you have produced, and are continuing to produce, any genuine, worthwhile fruit for Him.
Moving back, therefore, to the image of our being in the palm of God's hand, the point is that if you are a God-fearing, repentant, sincere, genuine person, you would not even want to be presumptuous, complacent, or over confident. If so you will not choose to throw yourself out of the palm of God's hand. Neither will you wander off, or show contempt for His forgiveness. On the contrary, you will want to produce fruit for Him.
The sort of person who is anxious for reassurance is like that precisely because he will not despise or become complacent about God's mercy and forgiveness. The very fact that it means so much to him, and that he so wants to be reassured, means that he is not likely ever to take it for granted.
Thus for such people, in those circumstances, it is true to say that they do 'have' eternal life. It is also true to say that no man can snatch them out of God's hand. They are safe and can feel secure. It would be like a refugee who is fleeing from a violent regime. Imagine it is the summer of 1939 and he has crossed the border, from Nazi Germany into France. He is now on a train heading to a port and then a ship to England. That refugee could ask "Am I safe now? Will this train definitely take me to the ship and to my new country?" The answer would be "Yes, you are secure. This train is definitely going to the coast and you are safe, You can be assured of that."
What that refugee really means by his question is "Will anybody make me get off the train against my will, or refuse to let me continue my journey?" He isn't focusing on whether he himself will choose to get off the train. That's not what he is worried about. He fully intends to stay on board. Thus, for Him, the assurance given is correct and meaningful.
That illustration can also help us to understand the warning passages and to realise what kind of people are being addressed and in what circumstances.