I tell you is similar in form and function to “Truly, I say to you” of verse 10. It both adds emphasis and serves as a transitional. The phrase can be translated as “Let me tell you” or “Let me assure you.” Jesus is still talking to the followers who are with him.
The many who will come from east and west are Gentile believers. In translation it may be necessary to say “many others who trust in God as this man does.”
East and west are not to be taken merely as references to opposing geographical directions; the meaning is rather “from all over the world” or “from everywhere.”
Sit at table (used again in 14.19) translates one word in the Greek text. The reference is to the coming Messianic feast, which is symbolic of the joys to be experienced in the kingdom of heaven (see 22.1-14; 25.10). It may be necessary to indicate that the reason these people will sit at table is to eat. Good News Translation has indicated this with “at the feast.” Other translations may have “sit to eat” or “come to feast (or, eat) with.”
Many readers will not know who Abraham, Isaac, and Jacob were. It may be necessary to say “our great ancestors, Abraham, Isaac, and Jacob.”
Kingdom of heaven was discussed in 3.2 and in chapter 5, and should probably be translated in a similar way here, perhaps as “when God establishes his rule.”
Bibel im heutigen Deutsch, 1st edition renders the entire verse: “But this is the way it will be when God has completed his work: many will come from the east and the west to sit at the table with Abraham, Isaac, and Jacob.” INCL renders “Many people will come from the east and west in order to rejoice together with Abraham, Isaac, and Jacob in God’s New World.” Another possibility is “Many people will come from all over the world to share in the feast with our great ancestors Abraham, Isaac, and Jacob when God begins his rule.”
Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on the Gospel of Matthew. (UBS Handbook Series). New York: UBS, 1988. For this and other handbooks for translators see here .