Hebrews 10:24-25, "And let us consider how to stir up one another to love and good works, not neglecting to meet together..."
Discipleship (and Sanctification) happens with the help of the Local Church, not without. The Holy Spirit works directly in our lives; the Holy Spirit works through our relationships; and the Holy Spirit works through our Local Church. Each of these is a biblical, and necessary, part of the Spirit's work: they are how the Spirit revealed to us that the Spirit works.
So, our Discipleship and Disciple-making has to be connected to the Local Church. This reflects the charge in Hebrews 10:24-25: do not stop meeting; stay in the program.
Why do our lives need to be connected to the Local Church? There are many ways to answer this question; here are a few.
First, the Local Church constrains our notions and actions by the Word of God. It's easy enough--we all do it--to have ideas about God, ourselves, or life, that are not biblical. It is also easy to slip into patterns of acting, thinking, speaking, that are contrary to the clear commands of Scripture and damaging to the Name of Jesus. The Local Church pulls us back into the orbit of the Word of God: songs, prayers, readings, meditation; even classes, counseling, and casual conversations. We need the multi-pronged appeals of Scripture. The Local Church delivers them to us.
Second, we need the constraint of "Polity." That is, membership and eldership and structure and schedules and order. The Christian Life is not all immersive worship experiences, loosing one's self in prayer and contemplation, or chasing passage after passage in in-depth Bible study. It's discipline, accountability, kindness-encouraging order, calendars, and plans. While Scripture contains glimpses of the joys of the former, it contains many commands regarding the latter too. It's a trellis-vine situation. The constraints of the Local Church make possible the flourishing of genuine spirituality and experiences of The Divine Glory.
Third, we need the constraint of Physical realities. It's easy--always has been, is, and always will be--easy to devolve the Christian life from a bread-and-wine, book-and-knee, voice-and-eyes-and-hands, widows-and-orphans, kind of life to a mystical, inner, isolated, and, frankly, selfish, "spirituality." After legalism, this was the second heresy the early church fought. But it's impossible to maintain that idea of Christianity when you have to vaccuum up animal-crackers after other people's kids, or when you're asked to hand the communion plates to someone you're not super fond of, or when you're asked to show up early and set up chairs, or when you find yourself serving a Christian brother or sister in humbling ways. Local Church life pops the mind-balloon of fanciful idealism and yanks it back down to the mud and spit of Christ. Those are what He used to heal the blind.
I attended a Bible College. I enjoyed growth and profound spiritual experiences. I enjoyed the solitude and immersion. But over the following decade, I found it had an unhealthy effect in my heart. It created a kind of nostalgic spirituality: my best days were my yesterdays.
While this kind of thinking rings true culturally--we all want what we had when we were 20--it should ring deeply false biblically. Maturity, like the fruitful oak that shelters whole ecosystems, is our ideal. We want to be maximally useful, not maximally self-indulgent. The Local Church is a staff, a schedule, a rhythm of services and relationships and sermons and check-ins and events, that are meant, over time, to not only sustain our faith through difficulties, but also to strengthen us, deepen our faith, and brighten our usefulness.
Your best days, believer, are ahead of you. While the rest of us gets saggy, our branches thicken, our roots deepen, our faith weathers, our fruitfulness becomes reliable, our reputation as a source for Jesus-love spreads. All this upon the trellis of the Local Church; all this in the root-knitted orchard of the Local Church; all this with the husbandry of the Local Church.
"The Love of Christ constrains us..." (2 Cor 5:14a) And not just in an inner-resolve sort of way, but through the external support the Spirit has called us to receive, in the life together that is the Local Church community and organization.