1. Scripture is key. It is the inerrant, infallible word of God that He breathed for select men to write His thoughts in order to give us a firm foundation. Without it we're just making guesses.
2. God is the primary point in Christianity. He is the reason. He is One, not multiple. He is a Trinity, which is not three gods or three modes, but one God in three persons, coequal, coexistent, distinct but not separate. Above all else, He is Holy.
3. Man is the primary problem in Christianity. We were made in God's image, but we are fallen and without hope. We stand convicted of Cosmic Treason against a Holy God and are in dire need of help.
4. Christ is God's answer to our problem. Jesus lived a sinless life, died on our behalf, and rose again. He was fully Man and fully God and fully real. There is no other answer available but faith in Jesus, the Christ, alone.
5. The Church is the Body of Christ, the Temple built of "living stones", necessary for believers. It is the gathering of the saints, those set apart by God, for the purpose of building one another up toward maturity.
6. Justification is by grace through faith in Christ. It is the imputation of our sin to Christ in return for the imputation of His righteousness to us. It is not earned, merited, or achieved. None of these are within the ability of Man to obtain. It is declared by God.
7. Sanctification is the necessary process by which God's saved children are transformed, changed necessarily through the leading and discipline of God in His own.
8. Glorification is the endgame, the final outcome. It is at the return of Christ in which our bodies are resurrected and transformed and we will be finally and completely joined to Christ.
There is one other concept that must be stated here simply because it has been refuted too often. It must be stated because it is essential. If it is not true, neither is Christianity. Hopefully by outlining these essentials you can see why it must be true or no Christianity.
Given that Scripture is clear on this and that God is Holy, given that Man is sinful and Christ is the only answer, given that justification is required and glorification is the end, there are two truths that must be true if any of this is to matter. These two are two sides of the same coin. They are the certainty of Hell for those who are not saved and the certainty of Heaven for those who are.
The Bible isn't even slightly hesitant or vague here. Jesus spoke repeatedly of eternal torment (Luke 16:28) and the fire that never goes out (Mark 9:47-48) for those who don't repent. It is the just reward for those who are not justified -- made right. Now, we can dispute over terms -- "Sheol" as the place of the dead or "Hades" as the eternal place of torment -- but we cannot avoid the reality of such a place. We can debate about the environment. Is it actual fire and physical torment, or something else? But we cannot avoid the fact that eternal torment is in view. Hell is not a story manufactured by mean-spirited folk intent on scaring people into being good. It is the correct, righteous outcome for those who intentionally violate the commands of the Most High. Anything less is not justice. Any claim to anything less is a failure to grasp the fundamental problem and magnitude of sin.
And the Bible isn't unclear about the outcome for believers. It is glorification, to be sure. But in that there is the certainty of a "new heaven and a new earth" (Rev 21:1). Jesus will reign among us (Rev 21:3). It will be a place of bliss where "He will wipe away every tear from their eyes, and death shall be no more, neither shall there be mourning, nor crying, nor pain anymore, for the former things have passed away" (Rev 21:4). A real Heaven.
Some have argued, mostly recently, that Hell is not a real place. There is no place of eternal torment. This serves to delete Scripture as a viable source and to mitigate sin as a genuine problem. In the end, Christ's death becomes pointless, unnecessary, if Hell is not real. Equally, all God's promises to His own are erased if Heaven is not a real place. Not a place with clouds and harps, but a place of joy where we are joined to Christ without sin or pain. Remove either of these, and you remove Christianity. But, oh, with all of these together, what a wonderful faith we enjoy!
Since we have a great priest over the house of God, let us draw near with a true heart in full assurance of faith, with our hearts sprinkled clean from an evil conscience and our bodies washed with pure water. Let us hold fast the confession of our hope without wavering, for he who promised is faithful. And let us consider how to stir up one another to love and good works, not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near (Heb 10:21-25).