Answer
The KJV and the NIV are different because they follow different Greek manuscripts on this verse. Most ancient Greek manuscripts do not include the phrase "take up your cross," but that phrase does happen to be present in the Received Text, which is the one from which the KJV was translated. Probably, the NIV is correct that the phrase is not original to the text.
In any event, Jesus makes similar statements in Matthew 10:38; 16:24; Mark 8:34 and Luke 9:23. In most of these contexts, Jesus also speaks of his death, so that the verses may hints at the type of death he is going to die (Mark 10:21 is also followed by a reference to Jesus death in Mark 10:33-34).
In those cases where Jesus' death is not mentioned, Jesus' words may be taken as metaphoric of the type of sacrifices his followers are required to be willing to make. Those who are not willing to die for him are not worthy of him. Even apart from martyrdom, life as a Christian involves suffering. This should have been understood by his original audience even if they had not known how he was going to die.
In those cases where Jesus' death is mentioned, it may still be true that Jesus intended his statement metaphorically, so that the meaning would be the same as in those cases where his death was not mentioned. However, it is tempting to see the mention of Jesus' death in close proximity as an indication that actual martyrdom was a real possibility for his followers.
For us and for the original readers of the gospel accounts, there is a greater significance in Jesus' words because we know the type of death that Jesus died. The image of the sacrifice that we might have to make is all the more significant because we know what they did to our Lord.
Answer by Ra McLaughlin
Ra McLaughlin is Vice President of Finance and Administration at Third Millennium Ministries.