If the first Christians were Jews, then those first Christians were obviously from one of the 12 tribes of Israel. If so then some Christians around the world should have Jewish linage from one tribe of the 12 tribes. Using this logic could a Christian today be from one tribe of the 12 tribes of Israel, making it possible for Christians to be among the 144,000 from the 12 tribes of Israel sealed in Rev. 7?
If when you’re speaking of Christians you mean Church Age believers, Galatians 3: 28 says that in the Church there is neither Jew nor Gentile. Therefore, regardless of his or her heritage no one from the church is regarded by God as still being part of Israel, and so none will be found among the 144,000 of Rev. 7. But even if they could be, when the 144,000 are commissioned the Church will already be gone in the rapture.
However, it’s important to remember that even though they’re from 12 tribes of Israel, all of the 144,000 will be believers in Jesus the Messiah.