And he marvelled because of their unbelief. And he went round about the villages, teaching. (Mark 6:6)
This is a good question which demands a good answer. However, it is far from overthrowing Calvinism as the author obviously thinks it will .
First of all, it is a marvel in itself (from our point of view) that Jesus himself marvelled at all. He is the Eternal God and therefore "knows all things" and knew all things even from before the foundation of the world. This was not something that sprung itself upon Him in total surprise. However, even with that being so, the expression is still there.
Secondly, that said, He did so in order that He might also teach us, by example, to see the depths of their wickedness that led to such unbelief.
Thirdly, it is written in Luke 7:9 that Jesus 'marvelled at the faith' of the centurion. Why was this? Faith is the fruit of His Spirit. The disciples had prayed that the Lord would increase their faith and, again, there is nothing that surprises Him. But just as the Lord did not believe for the centurion, so he did not doubt for the wicked in Mark 6:6. His marvelling is an indicator of the depths both of the centurion's faith and the wicked's unbelief. In Mark 6:6, the main teaching of the passage is the wicked unbelief of those who, having the Bible, should have known better.