What if someone is baptized for the remission of sins in order to become a member of the church of Christ…but does not know that baptism is a likeness of the death, burial, and resurrection of Christ when baptized (Romans 6:3-5)?

Good question.  On the one hand, the conversions recorded in Acts do not show that the ones teaching the converts before their baptism taught them Paul’s teaching in Romans 6:3-5.  (It should also be pointed out, however, that there is a clear indication on several of these instances that much more was said than what was recorded in Acts.  See Acts 2:40a and 8:35.)

On the other hand, it should be noted that when Paul brings this up to the Christians in Rome, he acts as if they should already know this:  “Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death?”  (Rom. 6:3, emphasis added).

In the end, all I can tell you is what I choose to do concerning this, which is to do my best to proclaim to potential converts in my personal studies with them the whole counsel of God concerning water baptism and its place in God’s plan to save our souls (cf. Acts 20:27; Ps. 119:160a).  In other words, we discuss not only Romans 6:3-5, but also Colossians 2:11-14, 1 Peter 3:21, John 3:3-5, Titus 3:3-5, 1 Corinthians 12:13, Mark 16:16, Matthew 28:19, the conversion examples in Acts, etc.  That’s just my own choice.  While I do recommend it, in the end what you do concerning your teaching on this matter is between you and God, as it is with all things in life.  I would say that if you do not bring up Romans 6:3-5 before you baptize them, make sure you teach them about it not long after you baptize them.