Those who place their trust in Him are, yes.
He was born of the tribe of Judah when He came in the flesh, yes.
None of these things make us of the tribe of Judah though.
Paul is explicit:
But on the contrary, when they saw that the gospel for the uncircumcised had been committed to me, as the gospel for the circumcised was to Peter(for He who worked effectively in Peter for the apostleship to the circumcised also worked effectively in me toward the Gentiles),and when James, Cephas, and John, who seemed to be pillars, perceived the grace that had been given to me, they gave me and Barnabas the right hand of fellowship, that we should go to the Gentiles and they to the circumcised. - Galatians 2:7-9 http://www.biblegateway.com/passage/?search=Galatians2:7-9&version=NKJV
For this reason I, Paul, the prisoner of Christ Jesus for you Gentiles—if indeed you have heard of the dispensation of the grace of God which was given to me for you,how that by revelation He made known to me the mystery (as I have briefly written already,by which, when you read, you may understand my knowledge in the mystery of Christ),which in other ages was not made known to the sons of men, as it has now been revealed by the Spirit to His holy apostles and prophets:that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel,of which I became a minister according to the gift of the grace of God given to me by the effective working of His power. - Ephesians 3:1-7 http://www.biblegateway.com/passage/?search=Ephesians3:1-7&version=NKJV
Christians are not part of Israel. We are a NEW creature. We're under a different covenant than Israel.
Therefore, from now on, we regard no one according to the flesh. Even though we have known Christ according to the flesh, yet now we know Him thus no longer.Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new.Now all things are of God, who has reconciled us to Himself through Jesus Christ, and has given us the ministry of reconciliation,that is, that God was in Christ reconciling the world to Himself, not imputing their trespasses to them, and has committed to us the word of reconciliation. - 2 Corinthians 5:16-19 http://www.biblegateway.com/passage/?search=2Corinthians5:16-19&version=NKJV