In the New Testament there were no “altar calls.” Instead, baptism was the public focal point of conversion in the early church. It served to protect against substituting mere intellectual assent for genuine faith, and it can serve the same function today.
If people tell you they already believe in Christianity, but you suspect something’s amiss, ask, “Are you willing to be baptized? Are you willing to stand up as an adult in public and confess your faith and trust in Jesus Christ, and to publicly identify yourself with Christ’s Church?” If they’re not willing, I suspect their so-called faith is just intellectual assent, not saving faith.
Willingness to be baptized helps separate the sheep from the goats, spiritually speaking. It distinguishes those who have a genuine faith in Jesus Christ from those who are merely giving intellectual assent to the doctrines of Christianity. True biblical faith entails action and identification. If one acts to publicly identify himself with Jesus Christ and with His people in baptism, then he’s probably moved beyond mere intellectual assent.