At one point we put out a video promoting our flagship event, CHALLENGE. One of the comments I received from that video was from a guy who argued that our comparison of military discipline to the Christian walk was invalid. He asserted that it was purely an Old Testament concept and therefore not applicable to the New Testament church. I gave counterexamples from II Corinthians and II Timothy to which he responded (I'm paraphrasing), "See! Those are Old Testament verses!" In fact, they are not. This guy needed to read his Bible more, starting with the table of contents.
If we skip the full knowledge that can be found in the Bible and reduce Christianity to "be nice," we shouldn't be surprised to find adult children in the pews and tragically even in the pulpits at times. Have a reading plan. Read your Bible. All of it.