A recent post on the ex-CoC board prompted a scriptural explanation of the necessity of physical baptism.
First, we can see a foreshadowing of baptism mentioned in the Old Testament...
There's Noah's ark, where they were saved through the use of water, the Israelites who walked through the Red Sea, Naaman who was cleansed by water...many prefigurations of salvation by some use of water. Also, Ezekiel 36:25–26: "I will sprinkle clean water upon you, and you shall be clean from all your uncleannesses, and from all your idols I will cleanse you. A new heart I will give you, and a new spirit I will put within you."
God made us physical AND spiritual creatures. It makes sense, then, that baptism is physical AND spritual. Remember that combination, water and spirit, physical and spiritual, as we look at other verses...
In John 3 verse 3, it says "Unless one is born anew, he cannot see the kingdom of God." 2 verses down, in verse 5, we get an explanation of what being "born anew" means. "Unless one is born of water and the Spirit, he cannot enter the kingdom of God." There's that necessary combination of water and spirit!
There's also Titus 3:5, "He saved us, not because of deeds done by us in righteousness, but in virtue of his own mercy, by the washing of regeneration and renewal in the Holy Spirit." Again, washing (physical) and renewal in the Spirit (spiritual).
Also, in the often-used Acts 2:38, we see this combination: "Repent, and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit." We see baptism (physical) and the reception of the gift of the Holy Spirit (spiritual.)
Now, are there times in scripture where only the physical or only the spiritual aspect is mentioned? Absolutely! But affirming one aspect of baptism doesn't indicate that the other aspect is unnecessary. It is clear from writings of early Church fathers that at the beginning of Christianity, both the physical and spiritual aspects of baptism were seen as equally important and completely intertwined, they were seen as united. It wasn't until relatively recently that the idea of symbolic baptism even gained popularity.
The bottom line is, when ALL scripture concerning baptism is considered and taken as a whole, we can easily see that water and the spirit are two aspects of one act, which are often mentioned together, and often mentioned seperately. Talking about the different aspects of baptism seperately, though, does not negate their necessary unity. We certainly could not have the physical without the spiritual and call it baptism - that would just be getting wet. Under normal circumstances*, we could not have the spiritual without the physical because, being physical creatures, God knew we would need the physical part of baptism to know that we have, in fact, received something spiritual since it is invisible. This is exactly why Christ gave us the sacraments, which are by definition outward signs of inward grace - the physical and spiritual united. This is what we're told to do over and over in the scriptures, and there's no reason to think they don't mean what they say.
Personally, I tend to take 1 Pet. 3:21 at face value when it says "Baptism now saves you."
*Certainly, extraordinary circumstances exist, such as baptism by blood and baptism by desire, where the physical aspect, water, is not used. But the existence of exceptions because of God's mercy does not change the normative way which Christ gave us to be added to His Church.