Upvote:2
Pope Benedict XVI wrote in the first article of his encyclical Spe Salvi.
“SPE SALVI facti sumus”—in hope we were saved, says Saint Paul to the Romans, and likewise to us (Rom 8:24). According to the Christian faith, “redemption”—salvation—is not simply a given. Redemption is offered to us in the sense that we have been given hope, trustworthy hope, by virtue of which we can face our present: the present, even if it is arduous, can be lived and accepted if it leads towards a goal, if we can be sure of this goal, and if this goal is great enough to justify the effort of the journey. Now the question immediately arises: what sort of hope could ever justify the statement that, on the basis of that hope and simply because it exists, we are redeemed? And what sort of certainty is involved here?
Anyway, I read your question and I immediately thought the title of the encyclical is probably the answer to this question.
Hope is what defines Catholics. It doesn't become the sin of presumption which would attempt to define what God can or can't do as the judge of our souls. But it shouldn't be despair either. Catholics never believe that anyone is too bad to be forgiven.
Catholics define that life within you as the life of grace, sanctifying grace in particular, which is our inheritance when we are made children of God at Baptism. But also actual grace, the grace to do good acts, which comes from works of prayer fasting and almsgiving.
This everlasting life is the same everlasting life that Jesus promised to the Samaritan Woman, the same everlasting life that comes from eating the Body and Blood of Jesus in worthily and in faith. This life of grace within us can be killed or severed (I don't know which is a better metaphor) by gravely sinful acts which is what Catholics call mortal sin.
John immediately after says there's a mortal sin that should be avoided if you want this life in you. So, I'm not going to speak for St. John and I don't know that the Church addresses this exact question in light of a notion of 'assurance', but it seems reasonable that what is important is knowing yourself, believing you can know when you're not mired in mortal sin, and the Church does afford us each this right (unless scandal is involved).