Christianity is other worldly, next worldly, in its nature.
But because Christians are resurrected into a Heaven of persons, this life is a necessary and potentially valuable prelude.
As a loose analogy: a doctor must first attend medical school. School is finite and much shorter than medical practice; but (If opportunities are grasped), that which is learned at school may have "permanent" and beneficial effects on the large future beyond.
Thus the benefits of medical school are best grasped when the student knows he is destined for a long professional practice.
Thus we are meant to be confident in our salavation, confident that we resurrect and go to Heaven after this life. Confidence is correct.
But we will go to Heaven only if we make the choice to follow Jesus, just as we will only become a doctor if we learn at medical school.
If we fall into bad habits at medical school, we will cease to want to become a doctor; and will drop out. Likewise, when our mortal life is wrong, we may choose to reject Heaven.
Otherwise, our own salvation is indeed assured.