I've been hearing a lot lately the idea that there was a gospel of the kingdom for the Jews only, and another message, a gospel of grace, for the Gentiles. It is held that the Twelve preached the former and Paul the latter. Baptism is a part of the plan of salvation, but only for Jews; Gentiles need only to believe, since the Twelve were apostles to the Jews, whereas Paul was an apostle to the Gentiles. The idea is that for the Jews to be saved, they must repent and be baptized (Acts 2:38), but for Gentiles salvation it is by faith alone -- the "revelation" to Paul mentioned in Galatians 3. Have you come across this teaching? -- Fenton Gardner

Yes, I have -- first in the 1980s, though I believe the idea is older. On once occasion I was preaching in Philippi (2006), and the text given to me as Acts 16:25ff. (the conversion of the Philippian jailer and his family). After I spoke, another preacher stood up, and tried to persuade the audience that repentance and baptism were unnecessary, since Peter's message applied only to the Jews living at the time of the birth of Christianity. 

Anyway, even though the notion has been out a long time, it lacks merit. Here's why:

  • The old covenant prophesied its own replacement (Jer 31; Isa 59). So there aren't two active, valid covenants now, but only one covenant, or testament (Gk. diathēkē = covenant), Heb 9:15-17.
  • As Paul explains in Galatians, his gospel (received by revelation from Christ) was the very same message that Jesus had entrusted to the Twelve. It is an oversimplification to create two theologies, since Peter preached to Gentiles (as in Acts 10) and Paul preached to Jews nearly wherever he traveled (Rom 1:16). 
  • The mystery Paul declares (Rom 16:25; Eph 3:3,4,6,9) is that the Gentiles are welcome into the family of faith -- and without needing to become Jews first (see esp. Galatians and Acts 15). But that hardly means they needn't repent, or can forgo baptism. Gentiles too are baptized (Acts 10:47; Eph 4:5) after repenting (Acts 20:21; 26:20; Rom 1:5; 2 Pet 3:9). John the Baptist required the Roman soldiers to repent (Luke 3:14). 
  • In Acts 2, Peter declared that the new era, the world to come, the Messianic age, was now dawning. In Acts 2:39 he seems to be saying that the promised Spirit, available through baptism (2:38) is for the Gentiles. (The term "far off" typically refers to Gentiles, who may or may not be "far off" geographically [from Israel], but rather "far off" theologically.)
  • Moreover, Acts 2:39 has "for all whom the Lord our God will call." It doesn't sound like there are any exceptions.
  • This is a Protestant overreaction to medieval works salvation. The rejection of baptism as a "work" is a novelty; the early church viewed baptism as part of the conversion process. 
  • Nowhere in the N.T. is salvation said to be through "faith alone." In fact the only place where this phrase occurs (Jas 2:24) seems to contradict Protestant theology. Recall that Luther rejected James as legitimate scripture -- for this very reason.
There you have it. There is but one people of God today, one covenant, and but one way to enter that covenant. I hope this response has been helpful. 
Somewhat related to this theology are the doctrine of the Mass Conversion of Israel, as well as the modern construct called Messianic Judaism.