How to reach a check point
I have strengthened all of them and still it is light grey
Actually, checkpoints are meant to be crossed. Before crossing the checkpoint, the colour will be golden/yellowish with an option to test out of some number of skills. It acts as a shortcut to jump ahead in the tree instead of doing each skill individually. Once all skills prior to a checkpoint are practiced, the checkpoint becomes meaningless and greys out.
You can read more about testing out here.