“Coexist” is a term from the unbeliever of the Biblical God, the coward and the lazy.
All scripture is God-breathed – therefore it matters not who wrote it, all of it comes from God through the authors God hand picked.
So who is going to heaven? Quite simple. Christians. True Christians. Who are the true Christians? You will know them by their works. Christians are followers of Christ and are such strong followers they congregate to talk about God every chance available.
A Christian studies scripture and is always fired up to talk about God and His word in the Bible. Should you come across a self-proclaimed Christian, test them, bring up the Bible. See what happens. The fakers really on their own works, their own interpretation and have no need for the Bible. They will reject it with an army of excuses.