Here’s Mark 3:13-14:
And he goeth up into a mountain, and calleth unto him whom he would: and they came unto him. And he ordained twelve, that they should be with him, and that he might send them forth to preach,
There are two ways that this passage can be interpreted:
1. The “whom” refers to the 12. So, Jesus calls the 12 and they come to him. Then he ordains them.
2. The “whom” refers to a larger group than the 12. Jesus calls this larger group and they come to him. Then, out of that group, he selects a smaller group of 12 and ordains them.
If you are just looking at Mark’s account, there is no way to prove whether #1 or #2 is the better interpretation.
Now, there is a general consensus that Mark was the first Gospel written and that Matthew and Luke had a copy of Mark handy when they wrote. So let’s look at how they record this event. Here’s Matthew:
And when he had called unto him his twelve disciples, he gave them power against unclean spirits, to cast them out, and to heal all manner of sickness and all manner of disease. Now the names of the twelve apostles are these; The first, Simon, who is called Peter, and Andrew his brother; James the son of Zebedee, and John his brother; (Matthew 10:1-2)
It is pretty clear that the best way to interpret Matthew’s account is that it agrees with #1 above. Now here’s Luke:
And it came to pass in those days, that he went out into a mountain to pray, and continued all night in prayer to God. And when it was day, he called unto him his disciples: and of them he chose twelve, whom also he named apostles; (Luke 6:12-13)
So Luke’s account agrees with #2 above. There are important implications to the fact that Mark wrote an ambiguous text and Matthew resolved the ambiguity in one direction and Luke resolved it in the other. One implication is that scripture is not infallible. Another is that Mark should not be interpreted, as it often is, by looking at Matthew and Luke in order to figure out “what Mark really meant.” Matthew and Luke may be unaware of, or may have intentionally chosen to convey something different from, what Mark intended. This in turn raises some interesting questions about what to do with the differences between the Gospels, and why it is that we have four not-entirely-reconcilable accounts of Jesus’ life in the first place. But that’s a topic for another post.