According to the Gospel of John, Jesus did not trust “man” at all - "because he knew all men, And needed not that any should testify of man: for he knew what was in man.” He knew the nature of the human condition. He, the spiritual man, did not need the material thought testifying on his behalf, because he knew the fickle nature of this man - a self-centered perception, lacking moral depth.
Later, Jesus declared that “no man receiveth his testimony. He that hath received his testimony hath set to his seal that God is true.” Again, the man which thinks it has everything already figured out is inadequate to receive what the Christ has to offer. However, he that is yearning for something more than “the ghastly farce of material existence” is open to the Christ, and "gets it" as evidence that God is true.
What might this mean to us? We ought to discern what is in man. Trying to convince a material thought of spiritual things only aggravates and has no upside. If Jesus could not do it, who are we to try. Be ready, however, to meet the need of the hungering and thirsting, giving no heed to how they are dressed, what their background, etc.
It is wise to remember that some of the most material thoughts are committed religionists. An atheist who is honest to his convictions, who despises the hypocricy of the religionist, who recognizes his own inadequacy, is far more likely to be receptive to the Christ than the self-satisfied religionist who is busy protecting his own beliefs. Those that sought to crucify the Christ were of the latter category - and they still are today!
Jesus knew what was in man - so should we, in the spirit of the following: "And we solemnly promise to watch, and pray for that Mind to be in us which was also in Christ Jesus; to do unto others as we would have them do unto us; and to be merciful, just, and pure." (6th Tenet of Christian Science)