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Is it possible to achieve a reasonable certainty that you are saved?
Yes. The gift of salvation …is a gift. We are saved by Jesus of Nazareth the Christ.
It is by the grace of God – though the death and resurrection of Christ. One initially receives such if one is adult via repentance…faith and baptism (tis grace…not something we earn). (An infant may receive such via the faith of the Church and baptism).
Now the question comes in the time following such – if one can have a ‘reasonable certainty’ (not an absolute certainty) that such remains the case?
Yes it is possible to have a kind of reasonable certainty that one living here and now in Christ…
And a firm hope that one will remain living in him when one leaves this world.
The certitude of hope is a very real thing.
"Moreover, our radical belonging to Christ and the fact that “we are in him” must imbue in us an attitude of total trust and immense joy. In short, we must indeed exclaim with St Paul: “If God is for us, who is against us?” (Rom 8: 31). And the reply is that nothing and no one “will be able to separate us from the love of God in Christ Jesus our Lord” (Rom 8: 39). Our Christian life, therefore, stands on the soundest and safest rock one can imagine. And from it we draw all our energy, precisely as the Apostle wrote: “I can do all things in him who strengthens me” (Phil 4: 13).
Therefore, let us face our life with its joys and sorrows supported by these great sentiments that Paul offers to us. By having an experience of them we will realize how true are the words the Apostle himself wrote: “I know whom I have believed, and I am sure that he is able to guard until that Day what has been entrusted to me”; in other words, until the Day (II Tm 1: 12) of our definitive meeting with Christ the Judge, Saviour of the world and our Saviour."
~Pope Benedict XVI
(from:
vatican.va/holy_father/benedict_xvi/audiences/2006/documents/hf_ben-xvi_aud_20061108_en.html)