Matthew 16:18 (ESV)
18 “And I tell you, you are Peter, and on this rock I will build my church, and the gates of hell shall not prevail against it.”

A believer can be 100% for sure that the Holy Spirit is personally and actively “building” them into the Body of Christ with other believers.

1 Peter 2:4-5 (ESV)
4 “As you come to him, a living stone rejected by men but in the sight of God chosen and precious,
5 you yourselves like living stones are being built up as a spiritual house, to be a holy priesthood, to offer spiritual sacrifices acceptable to God through Jesus Christ.”

The Spirit builds up living stones.

The living stones respond to the Spirit’s building.
1) Discern the building direction of the Spirit.
     (Who is He orchestrating you to connect with?)
2) Intentionally step by faith into the Spirit’s building.
     (Practically set time and context for the Spirit to “mortar” you with other living stones.)
3) Stay pliable in the hands of the Master Mason.
     (Let him place and re-place you as He wishes.