I am telling you the truth is the same expression used in verse 16. (On the meaning of this expression, see 1.51.)

This verse, like many other passages in the Gospel of John, affirms the unity of Jesus with God the Father. At the same time, it affirms the unity of Jesus and his disciples, so that whoever accepts his followers accepts him and so accepts God. In this context receives may be understood in the sense of “to welcome.” In some languages one may even say “welcomes into his heart.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .