In John 5:31 Yeshua says:
If I bear witness of myself, my witness is not true.
If the second person of the triune god bears witness of himself, why is it not true?
First, lets look at the setting of the entire passage. Jesus was talking to Jewish leaders, and they were accusing him for his claims of equality with God (John 5: 17- 23). This verse has to be looked in a historical and cultural context. In the Jewish law, the testimony of a witness is not received in his own case, so these leaders would render his claim, although true, to be null in court. So Jesus is adhering to their principles: you needed to have other witnesses (Deut. 19:15). Even Jesus advocates for this in Matthew 18:16. However, its obviously true because there are other witnesses. He has the Father (John 5: 32) (John 5: 37), the witness of his cousin John the Baptist (John 5:33), the witness of his own works (John 5:36) and of the Scriptures and Moses (John 5: 39, John 5:45).
Edited on permission: It has far more to do with His audience than His nature. He goes on in the following verses to illustrate how He has satisfied the Jewish legal requirements of witnesses. Take note as well that ascribing actions to one of Christ's natures or the other can quickly evolve into Nestorianism. If that is your position, that's fine. But you should be aware that it is not consistent with the vast majority of historic Christianity. - @brandimus (local source)
My answer is heavily influenced by "The Robertson's Word Pictures of The New Testament and a user to this site by the name of @brandimus. Another analytic source you can use is "John Gill's Exposition of The Whole Bible, in which you can read more in depth on why Christ said this.