I read Baptized but Not Converted on your site, but I have a more specific question.
After a long study with some ladies, they all came to the conclusion that baptism was essential for salvation. One of the ladies, right before she was baptized, indicated that she wouldn't be attending services. Should she have been baptized at that point? Was her baptism scriptural? Sadly, all of the women now attend denominational churches.
In our eagerness to lead souls to Christ, we can become so focused on convincing a person of the need to be baptized that we forget that there is more that needs to be taught.
After tell the crowd at Pentecost that they needed to repent and be baptized, the Scriptures record, "And with many other words he solemnly testified and kept on exhorting them, saying, "Be saved from this perverse generation!" So then, those who had received his word were baptized; and that day there were added about three thousand souls" (Acts 2:40-41). Have you ever wondered what other topics were discussed in those "many other words?" When the jailer asked what he need to do to be baptized, Paul and Silas told him, "Believe in the Lord Jesus, and you will be saved, you and your household." But it didn't stop there. "And they spoke the word of the Lord to him together with all who were in his house" (Acts 16:30-32).
Jesus said, "For which one of you, when he wants to build a tower, does not first sit down and calculate the cost to see if he has enough to complete it? Otherwise, when he has laid a foundation and is not able to finish, all who observe it begin to ridicule him, saying, 'This man began to build and was not able to finish.' Or what king, when he sets out to meet another king in battle, will not first sit down and consider whether he is strong enough with ten thousand men to encounter the one coming against him with twenty thousand? Or else, while the other is still far away, he sends a delegation and asks for terms of peace" (Luke 14:28-32). There are two costs that need to be considered before becoming a Christian. You have to consider whether you are going to be able to complete the journey before you start it. I try to have a discussion with people who indicate that they want to be baptized about the fact that they are entering a permanent covenant with God that they will have to work on for the rest of their lives. The second cost is just as important to discuss: What will be the result if you don't yield to God's authority?
The woman's question indicated that she wasn't prepared to pay the cost. She wants to follow Christ but only on her own terms. This is why she immediately returned to denominationalism. If faced with a similar situation, I guess my first question would have been, "Then why do you want to get baptized if you don't plan on serving Christ?"
The problem that you and I have is that we can't read other people's minds. We can't determine their motivations. You teach to the best of your abilities and you are still going to run into those who want to be baptized without converting to true Christianity. Every preacher I know has stories of those they lost right after baptizing them. "The one on whom seed was sown on the rocky places, this is the man who hears the word and immediately receives it with joy; yet he has no firm root in himself, but is only temporary, and when affliction or persecution arises because of the word, immediately he falls away" (Matthew 13:20-21).
Teach more than just baptism. Teach what it means to be a Christian and what is expected of a Christian. Talk about the change in life that must occur. Tell about the benefits and the hardships that might arise. And know that even with your best effort, you are going to lose some. "If any man's work which he has built on it remains, he will receive a reward. If any man's work is burned up, he will suffer loss; but he himself will be saved, yet so as through fire" (I Corinthians 3:14-15).