My English version reads "truly, truly", which is to say he is speaking the truth, and that one must eat the "Word made flesh", which is the Word, the Law and the prophets, being fulfilled, by the power of Spirit, which dwells in the blood, or in a physical sense, drink the representative wine. Note that Yeshua said he would not drink the "new wine" until he returned, which is to say the "new covenant" of Jer 31:31-33, would not be fulfilled until he returned, which is at the end of the age, and is represented by the time when Israel would be gathered from the nations and united with Judah (Ez 36 & 37), when Israel would get a new heart and a new Spirit. Saying that what one says is true, is not an oath. What he saying is it is true that one must eat the bread of life, the Word, and drink the power behind it, which is the Holy Spirit, or you will remain among the walking dead. To enter into life, one must keep the Word/commandments (Mt 19:17).