Writing for different groups result in different materials. Imagine writing a systematic theology for an unbeliever, then one for the typical Christian, then one for your closest apprentice, or even your wife. All three would have value, and they would be very different. The unbeliever might show some appreciation for the first version, and probably less with the other two. The apprentice would benefit from all three, but in the long run he would want to read the third one over and over again. If you could write only one, you might wish to write the third one, because you could pour all your heart into it, but you should probably write the second one, since it would be the most versatile.
There are some things you would not say to an unbeliever, or say as much, not because of hesitation on your end, but because he is not ready to listen. You can say more to a Christian, but sometimes he is barely better than the unbeliever, and it is counterproductive to tell him everything you know (Matthew 6:6, Mark 4:34, John 16:12). As another example, I would not write a system of argumentation for the general public that contains my unrestrained thinking on the subject, not even for the Christian. He does not need it to defeat every unbeliever in the world, and he would probably use it to bully his wife anyway. But I can say more to a trusted apprentice.
Most Christians are a drag. Forget about making them run and fly — you need to evangelize them! Forget about teaching them the strongest principles in applied apologetics — they are still whining about methodology. However, with someone who is of one mind, one faith, and one Spirit with you, you can share all your thoughts and experiences with God. When faith speaks to faith, there is no holding back, no preemptive qualifications, no tedious polemics, but only the building up of one another from faith to faith, from glory to glory, attaining to the whole measure of the fullness of Christ. This is what we ought to strive for in our own circles of believers. Look around you. If you are just drowning in religious otaku, then instead of continuing to waste your life for their approval, it would be better to come out from the illusion of piety.