Why do some denominations say that you have to be fully immersed in water, or you are not truly baptized?

This is a great question and, sadly, one that has kept many Christian brothers and sisters at odds with one another for far too long. Let's start with the definition of the word baptism itself. To baptize is to immerse. The idea, strictly speaking, is that one is immersed into another substance. Taking this definition into the Bible, we find John the Baptizer saying this, "I baptize you with water, but He will baptize you with the Holy Spirit," Mark 1:8. 

Here, we see a clear and important distinction. John the Baptizer, who is the last of the prophets is baptizing with water for repentance. Moreover, he is NOT administering Christian baptism. His baptism precedes Jesus and is more properly associated with the Old Covenant, not the new. Remember, he is the "forerunner" of the Christ. He has come to prepare the way for him. Under the New Covenant, Jesus will baptize with the Holy Spirit (and with fire). Thus, in the former case and under the Old Covenant, John is immersing people into water for repentance. In the latter case and under the New Covenant, Jesus is immersing people with the Holy Spirit.

Now, we see the picture coming into sharp relief. The concern of baptism in the Christian Faith is far more about the Holy Spirit than it is water. In other words, spiritually speaking, what good would it do for us to be washed with water on the outside - our physical bodies - if we haven't also been washed in our spiritual constitution on the inside, by the regenerating work of God the Holy Spirit, for the forgiveness of sins, our adoption into his family, the empowerment for Christian ministry, and the infusion of hope unto eternal life?

If we take this to the extreme example, the point will become clear. Imagine someone on his death bed who wants to confess Jesus Christ as Lord and Savior for the salvation of his soul, but may neither be in the physical condition to be immersed or, on the other hand, nor is (enough) water available for immersion. Is he disqualified from salvation? Have the limitations of our human circumstances now put God in shackles, preventing His ability to impart the grace required to the man unto salvation?

Therefore, while immersion into water during the administration of the sacrament of Christian baptism is a wonderful thing, and should be offered whenever possible and celebrated with joy as it is done, the greater and more necessary, essential, and critical concern is that Jesus immerses us into salvation by the Holy Spirit coming upon us!

Thanks for asking and keep the good questions coming!

Fr. Chris Culpepper