They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us. - 1 John 2:19 (ESV)Not complicated, I know. But the truth is much often simpler than some like to admit. The fact is, the reason millennials are leaving the church is because they are unconverted. All the hand-wringing over what we could be doing differently in order to better disciple these young adults is, then, misguided because what they need, frankly, is not discipleship but evangelism. Yes, the Evangelical church has failed these young people. But the failure was not failing to cater to their every (unbelieving) whim or offer them the newest, hippest programs or whatnot. No, the failure is a failure to preach the gospel clearly and consistently. It is a failure to hold church members to biblical standards, a failure to practice biblical church membership and discipline.
Christ's sheep follow him, for they cannot fail to hear his voice (John 10:3-5). They grow in holiness (Ephesians 5:25-27; Hebrews 12:14). They do not forsake assembling together (Hebrews 10:25). They persevere to the end (Romans 8:29-30; Hebrews 3:14). The millennials are leaving the flock because they are not part of the flock. It's as simple as that, really.
*I do not mean to imply, however, that no one has offered a sound response, just that there seems to be a reluctance to offer a simple, straightforward response grounded in God's Word. For an excellent, biblically-faithful response, see here.