Please explain how he could die under the New Testament when the Old had yet to be fullfilled?
The Temple was not yet destroyed, Christ had not yet risen, the Devil had not yet been confronted in Hell.
what is clear from mark 16:16 is that both are necessary however baptism will not save apostates.
the reason st john the forerunner is in the Bible is the need for both baptisms.
But John says differently...
Luk 3:16 John answered, saying unto them all, I indeed baptize you with water; but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Ghost and with fire:
The curtain in the Temple had been ripped from top to bottom showing that access to God no longer was limited to a born priesthood.
Christ finished the requirements of the Law by his death and the thief died after Christ.
Apollos was preaching the Baptism of John the Baptist and was corrected by Aquilla and Priscla in Acts 18.
It was the wrong Baptism.
The Christian Baptism is found in Mat.28:19.
Spiritual baptism (which is what identifies the believer with Christ spiritually) is found in 1Cor.12:13).
Water Baptism is only a symbol of that spiritual baptism.