Let's turn to Scripture for the answer to that question! Scripture teaches that one’s final salvation depends on the state of the soul at death. As Jesus himself tells us, "He who endures to the end will be saved" (Matt. 24:13; cf. 25:31–46). One who dies in the state of friendship with God (the state of grace) will go to heaven. The one who dies in a state of enmity and rebellion against God (the state of mortal sin) will go to hell. Regarding the issue of whether Christians have an "absolute" assurance of salvation, regardless of their actions, consider this warning Paul gave: "See then the kindness and the severity of God: severity toward those who have fallen, but God’s kindness to you, provided you continue in his kindness; otherwise you too will be cut off" (Rom. 11:22; see also Heb. 10:26–29, 2 Pet. 2:20–21).
One can be confident of one’s present salvation. This is one of the chief reasons why God gave us the sacraments—to provide visible assurances that he is invisibly providing us with his grace. And one can be confident that one has not thrown away that grace by simply examining one’s life and seeing whether one has committed mortal sin. Indeed, the tests that John sets forth in his first epistle to help us know whether we are abiding in grace are, in essence, tests of whether we are dwelling in grave sin. For example, "By this it may be seen who are the children of God, and who are the children of the devil: whoever does not do right is not of God, nor he who does not love his brother" (1 John 3:10), "If any one says, ‘I love God,’ and hates his brother, he is a liar; for he who does not love his brother whom he has seen, cannot love God whom he has not seen" (1 John 4:20), "For this is the love of God, that we keep his commandments. And his commandments are not burdensome" (1 John 5:3).
Likewise, by looking at the course of one’s life in grace and the resolution of one’s heart to keep following God, one can also have an assurance of future salvation. It is this Paul speaks of when he writes to the Philippians and says, "And I am sure that he who began a good work in you will bring it to completion at the day of Jesus Christ" (Phil. 1:6). This is not a promise for all Christians, or even necessarily all in the church at Philippi, but it is a confidence that the Philippian Christians in general would make it. The basis of this is their spiritual performance to date, and Paul feels a need to explain to them that there is a basis for his confidence in them. Thus he says, immediately, "It is right for me to feel thus about you all, because I hold you in my heart, for you are all partakers with me of grace, both in my imprisonment and in the defense and confirmation of the gospel" (1:7). The fact that the Philippians performed spiritually by assisting Paul in his imprisonment and ministry showed that their hearts were with God and that it could be expected that they, at least in general, would persevere and remain with God.
We can, if our lives display a pattern of perseverance and spiritual fruit, have not only a confidence in our present state of grace but also of our future perseverance with God. Yet we cannot have an infallible certitude of our own salvation. There is the possibility of self-deception (cf. Matt. 7:22-23). As Jeremiah expressed it, "The heart is deceitful above all things, and desperately corrupt; who can understand it?" (Jer. 17:9). There is also the possibility of falling from grace through mortal sin, and even of falling away from the faith entirely, for as Jesus told us, there are those who "believe for a while and in time of temptation fall away" (Luke 8:13). It is in the light of these warnings and admonitions that we must understand Scripture’s positive statements concerning our ability to know and have confidence in our salvation. Assurance we may have; infallible certitude we may not.
One sacrifice for sins, offered once and it is forever.
That's what Scripture says. In the Dispensation of the Grace of God. Which is where we are today.