Hi yuehan,
Haha...thanks. It is humorous to look back on it. At the time, I was really wrestling with it.
I don't think that there are any passages in Scripture that explicitly refer to baptism as a "public declaration of faith." I think that it can be implied when you consider all of the passages dealing with baptism. Regardless, it is certainly an outward act that symbolizes something that has already taken place.
Without the symbolism, observance aspects, a person to actually "baptize" you and, of course, spiritual obedience, a "baptism" would be no different than bathing or swimming.
In most cases of baptisms in the Bible, they took place with people around. We know that the Lord commissioned us to "baptize" (Matthew 28:19). This obviously implies that someone is baptized by someone else. In virtually all cases of baptism described in the New Testament, there are people gathered together. The only case that I can find where there was possibly no one else around was when Phillip baptized the Ethiopian eunuch (Acts 10:26-40).
When the Ethiopian eunuch had the passage from Isaiah (and the good news) explained to him, he questioned what would prevent him from being baptized. They both went into the water and Phillip subsequently baptized the man (and was was taken away by the Lord to another area). We don't know if anyone else was around. They were simply in some water along the road between Jerusalem and Gaza (Acts 10:26, 36).
I know that some New Testament scholars will argue that baptism replaced circumcision as the sign of the covenant (in this case, a NEW covenant) between the Lord and his people. This is usually reinforced by Colossians 2:9-15.
This passage implies that baptism is a symbol of the death and resurrection of Christ -- being "buried" into the waters of death and then rising from it. This apparently rules out the "sprinkling." I find it interesting that the baptism described here is a symbol of our Lord's death and resurrection even though John had been baptizing with water years before the cross.
This is an interesting point about baptism. When the Pharisees confronted Jesus and questioned him about his authority to teach, Jesus turned the tables on them. In Matthew 21:25, Jesus asked them, "The baptism of John, whence was it? from heaven, or of men?" The Pharisees were caught in a dilemma. They reasoned that if they responded that it was from God, Jesus would ask, "Why did ye not believe him?" In other words, they implied that Jesus held that baptism was from God.
There are several extra-biblical historical descriptions about baptism too. From the Didache to Tertullian to Cyprian, there are several historical references to baptism from the first few centuries of Christianity. While such accounts certainly cannot be esteemed as canon or truth (and may very well be early perversions of the faith or the amalgamation of Christianity with first century Judaism [or, as with the Didache, possibly the works of Judaizers]), it does seem that it was viewed historically as something of a public declaration or symbol.
In a sense, the act of baptism might be like the act of marriage. A bride and groom can get married alone by a preacher or even public official (or, in the first century, by agreeing in contract via a ketubah or mohar); however, a marriage ceremony and/or breakfast/dinner is usually done as a public symbol of the joining of the bride and groom in matrimony.
I suppose that no one can say for certain that it is a "public declaration" -- but it certainly can be (and often is). Of the baptisms mentioned in the New Testament, only the one in which Phillip baptized the eunuch was possibly done with no one else around (and we don't know for certain if they were alone).
I think that the idea of baptism from Acts 10:44-48 is what really convinced me of the need for baptism -- even though I had already come to know the Lord (and was baptized with the Spirit).