“Jesus answered them and said, ‘Most assuredly, I say to you, you seek Me, not because you saw the signs, but because you ate of the loaves and were filled. Do not labor for the food which perishes, but for the food which endures to everlasting life, which the Son of Man will give you, because God the Father has set His seal on Him’ ” (John 6:26-27).
The need to be sincere in our spiritual pursuits is a command that reverberates throughout the New Testament. Of all the desirable traits we are taught in scripture, sincerity is one that really stands out.
The context of this passage is when Jesus teaches the disciples on the mountain side and then moves to another location. The disciples are amazed and question Him about how he arrived at the new location.
Jesus takes advantage of this opportunity to rebuke them because of their lack of sincerity, pointing out that their motivation for following Him is not acceptable—it is for material gain instead spiritual enhancement.
It is a mockery of God for us not to serve Him out of a true heart—we must really mean it when we profess our devotion to God and Jesus and when we perform Christian service, or it will not do us no good in eternity.