Some thimk accepting Christ as Savior and escaping hell is the emd of the transaction. It is only the beginning.
Being a Christian is much more than "fire insurance".
The "building" would be serving God in the fullness of The Holy Spirit, seeking Him with all our heart, mind, soul and strength, and growing closer to Him each day, preaching the good news of Christ and winning souls, and walking in holiness.
Thank you for asking.
P.S.
Catholics can and do build upon the foundation of Christ as I described above, but in my experience it was never taught as such Sunday mornings, and those who do don't walk around with a giant Catholic chip on their shoulder, they simply preach Christ.