Given that there are only two places to spend eternity, Heaven and Hell, and nothing in between, it follows that at the most fundamental level there are only two eternal spiritual states before God: condemned and justified (or not-condemned). (Jn 3:18)
Given that we all start out alienated from God, dead in sin, under the wrath of God (Ep 2:1-3), such that we’re all commanded to repent and believe (Ac 20:21), in order to go to Heaven a transition must occur when we go from being condemned to being justified. According to Scripture, this transition happens as we believe on Jesus Christ (Jn 3:36, Ro 3:26), so it is important to understand what this means, to ensure that we are justified and are no longer condemned.
The first thing we might notice is that if there are only two possible states before God: condemned and justified, any transition between these two states must be instantaneous; it must happen in an instant. In other words, in order to go to Heaven there be an instant in time when we stand condemned before God, dead in sin, headed for Hell, and the second after this instant we are justified before God, headed for Heaven, eternally safe, such that God will never again impute sin to us. (Ro 4:6-8) Becoming justified cannot occur gradually over a measurable period of time, or involve an ongoing process of growth and transformation. This is directly implied by the fact that there are fundamentally only two possible, eternal spiritual states: condemned and justified; there is nothing in between, no middle ground for us to occupy, even for a moment.
A second thing we might notice is that at the instant we are justified we must believe something new and different about Christ that we have not believed before, and this belief will relate in some way to the person, character and/or atoning work of Christ. (Ro 4:23-25) This follows from the fact that if we are justified by believing on Christ, justification is conditional upon having this belief in Christ, and having this belief in Christ implies that we are already justified. So, this belief in Christ must first occur in us at the instant of justification, and not sooner or later: justification happens as and when we believe on Christ, and we believe on Christ as and when we are justified.
This may seem trivial, stating the obvious, something anyone could discover by thinking just a little bit and using some common sense. Yet this understanding of salvation is very uncommon among professing Christians: that [1] there must be an instant of conversion (or salvation), and that [2] this event is marked by believing something new about Christ which was not believed before. (Ro 3:22)
Most all Christians do not believe salvation occurs when their beliefs about Christ and His work change. In most every case, what is believed about Christ just before the instant of salvation is identical to what is believed about Christ immediately afterward. In other words, no beliefs in or about Christ change at the point when most think they’re being saved. This is how most every evangelical gospel tract presents the good news and evidently how most every instructed Christian would explain it, basing salvation itself on something other than simply believing on Christ. This is — obviously — no small thing.
The typical substitute today, at least in Evangelical Christianity, is some form of The Sinner’s Prayer, in which a person, believing they are currently headed for Hell, and having been told the good news about Christ’s death, burial and resurrection and having believed the message, yet perceive nothing in the message itself suggesting that they are justified before God merely by believing it. Rather, they hope to experience this transition from being condemned to being justified by telling God they’re sorry they have sinned, asking Christ to save them, and committing their lives to serve and obey God. They think of this act of prayer as trusting Christ to save them, but nothing about what they believe is changing during this act of praying, so this cannot be the correct way to present or understand the gospel if salvation is by believing in Christ.
In the Bible, when the gospel message is presented, people are saved as they hear and believe the message; something miraculous happens within them causing them to believe on Christ (Ac 8:35-37); they are never encouraged to pray any kind of prayer or engage in any kind of ritual in order to be justified — they just believe. (Ac 10:43-44)
Praying to receive Christ does not get us to Heaven; neither does being baptized, taking a sacrament, following any religion, creed or tradition, or performing any kind of ritual. (Ga 6:15) Those who depend on such Man-made devices for their eternal safety have perverted the gospel of Christ and do not rightly understand it. (Ga 1:6-8)
It should come as no surprise that many — perhaps almost everyone — calling Jesus Christ Lord will be cast away from Him, as He pronounces those dreadful, final words: “I never knew you; depart from Me, ye that work iniquity.” (Mt 7:22-23)