I am telling you the truth is used a number of times in John’s Gospel. For its first occurrence see 1.51.

No slave is greater than … no messenger is greater than is literally “a slave is not greater than … a messenger is not greater than….” The word translated messenger appears only here in John’s Gospel: it is the normal word for “apostle,” but all translators recognize that it does not have that technical sense here. This verse is similar to Matthew 10.24: No pupil is greater than his teacher; no slave is greater than his master. The meaning is obvious: the disciple is not to feel too important to perform the acts of service that his Lord performed, nor is he to expect better treatment from the world than his Lord received.

In this context greater may be understood in the sense of “more important,” for example, “No slave is more important than his master.” Comparison may be expressed in some languages as involving a surpassing quality, for example, “No slave surpasses his master in being great.” For other languages a positive-negative contrast may be helpful, for example, “No slave is great while his master is not great.”

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 .