Actually no. It was revealed in clear speech through the prophets centuries before water baptism. The mechanism of salvation wasn't known, but anyone with a believing heart would have known gentiles would be saved. But this too has nothing to do with pedo vs, credo, so let's not sidetrack the discussion.
Yet the number of the children of Israel shall be as the sand of the sea, which cannot be measured nor numbered; and it shall come to pass, [that] in the place where it was said unto them, Ye [are] not my people, [there] it shall be said unto them, [Ye are] the sons of the living God. Hosea 1:10