Mark would fully understand being immersed into the body by the Holy Spirit at the time of writing Mark ( his mini summary of Acts in 16 towards the end. ) Thats why the later part of 16.16 - says but he that believeth not shall be damned.
Yes. Mark did accompany Paul and would have been privy to the revelation of the Church as the Body of Christ. I understand Mark 16:16 as water because of five other references to what Water Baptism does, and because of John 3:3-5 and 1st Corinthians 10. The main meaning of water immersion is the killing and burying of the old and unworthy flesh and the conscience. The immersion into the Holy Spirit is not to be saved, but to empower one for the ministry (Lk.24:49, Act.1:8). Here are the five:
- It is given to all God's people to pass through a flood to leave the old behind - Noah, Abraham, Israel and the Church (Josh.24:2, 14)
- It is the replacement for circumcision because we have been made seed of Abraham and heirs to the Promise (Gal.3:29, Col.2:10-12)
- It is the burying of the old man of sin (Rom.6:1-5)
- It is to clear the conscience (1st Pet.3:21)
- It is so that we are like Christ in resurrection (Rom.6:5)
Of course, I do not believe that Baptism has anything to do with the effects of rebirth. The Bible does not entertain any thoughts of rebirth by any other method than the Holy Spirit. Israel are already "sons" of Abraham when they cross the Red Sea and Jordan. We are "sons" of God already when we are immersed in water.
And also of course, the matter is deeper than my few lines here. The grand type of Baptism is the Laver of the Tabernacle and/or the Temple. It's purpose and situation in the outer court reveal much.
I am aware that this matter of Baptism is complicated, and appreciate that you might have some differences. That why we discuss here.