“Ever since I became a Christian I have thought that the best, perhaps the only, service I could do for my unbelieving neighbours was to explain and defend the belief that has been common to nearly all Christians at all times.”
Mere Christianity was the name that Lewis borrowed to describe the common beliefs held by “nearly all Christians at all times.” But again, where did he find the actual name, “Mere Christianity?” He got it from another my hero of mine, Richard Baxter. But what exactly was Baxter thinking of when he coined that phrase? J.I. Packer helpfully comments that by it Baxter meant:
“historic mainstream Bible-based discipleship to Jesus Christ, without extras, omissions, diminutions, disproportions, or distortions…”
Packer says that this was the faith that Baxter embraced and “sought to spread.” And for you Lewis fans, Packer goes on to say that “Lewis and Baxter belong together as men with a common purpose as well as a common faith.”
 … the holy Scriptures, which are able to make you wise for salvation through faith in Christ Jesus.  All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness,  so that the man of God may be thoroughly equipped for every good work.
God has provided for us in Scripture that which will make us wise for salvation. So too, God’s Word is useful for teaching the truths of the faith, rebuking and correcting us when we go astray, (both doctrinally or practically), and helping to train us to live lives that are pleasing to God. And so, getting back to my earlier point, as we believe and embrace God’s Word (or teaching), and faithfully live obedient lives as a result of His truth, then the watching world, by hearing what we believe and seeing that our lives match our confession, will be more attracted to the faith, and, maybe, desire to become a part of it. Could God draw folks to himself another way? Of course. But he has ordained that we serve as a means to His end. Moreover, what we find in these words by the apostle Paul is reflection of what our Lord taught. For example, Jesus tells us in John 8:31-32 that,
…If you hold to my teaching, you are really my disciples.  Then you will know the truth, and the truth will set you free.”
He also tells us in the Great Commission to “go and make disciples.” How? By “teaching them everything Jesus has commanded.” I want to suggest that Jesus’ teachings and commands are what constitute “mere” Christianity – that is, who he is, what he taught, what he did, etc., as well as how we’re to respond, etc. His teachings (including his teaching that comes to us throughout all of holy writ) are those non-negotiable essentials that each and every person who would call himself or herself a follower of Jesus Christ must commit themselves to believing and living out. Christians do so, not because it will save us, but because our Lord has commanded us to do so. Attention to our doctrine and life will be the light by which unbelievers will see us and praise our Father in heaven (Matt. 5:16). Because we have been saved, our obedience naturally and gratefully flows from our new position in Christ. Our fruit will determine what kind of tree we are.
“I like to hear a man dwell much on the same essentials of Christianity. For we have but one God, one Christ, and one faith to preach; and I will not preach another gospel to please men with variety, as if our Saviour and our gospel were grown stale–For it is the essentials and common truths, as I have often said, that we daily live upon as our bread and drink. And we have incomparably more work before us, to know these better, and use them better, than to know more. The sea will afford us more water after we have taken out a thousands tons, than a hundred of those wells and pits from whence we never yet brought any.”