Hi bite me,
I checked this verse on bible.cc. Most, but not all translations use 'of Christ' or 'of the Christ' According to the Greek lexicon on bible.cc 'Christos' meaning 'Christ' is used in the Greek text, so I don't know why some translations leave it out, maybe it depends what manuscript they use? Not sure, I'm not an expert! Here's some other translations of the verse:
New American Standard Bible (©1995)
For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek.
GOD'S WORD® Translation (©1995)
I'm not ashamed of the Good News. It is God's power to save everyone who believes, Jews first and Greeks as well.
King James Bible
For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
American Standard Version
For I am not ashamed of the gospel: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
Bible in Basic English
For I have no feeling of shame about the good news, because it is the power of God giving salvation to everyone who has faith, to the Jew first, and then to the Greek.
Douay-Rheims Bible
For I am not ashamed of the gospel. For it is the power of God unto salvation to every one that believeth, to the Jew first, and to the Greek.
Darby Bible Translation
For I am not ashamed of the glad tidings; for it is God's power to salvation, to every one that believes, both to Jew first and to Greek:
English Revised Version
For I am not ashamed of the gospel: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
Tyndale New Testament
For I am not ashamed of the Gospell of Christ, because it is the power of God unto salvation to all that believe, namely to the iewe, and also to the gentile.
Weymouth New Testament
For I am not ashamed of the Good News. It is God's power which is at work for the salvation of every one who believes--the Jew first, and then the Gentile.
Webster's Bible Translation
For I am not ashamed of the gospel of Christ: for it is the power of God to salvation to every one that believeth; to the Jew first, and also to the Greek.
World English Bible
For I am not ashamed of the Good News of Christ, for it is the power of God for salvation for everyone who believes; for the Jew first, and also for the Greek.
Young's Literal Translation
for I am not ashamed of the good news of the Christ, for it is the power of God to salvation to every one who is believing, both to Jew first, and to Greek.