Paul was surprised and astonished that people could walk away from faith so easily and start to chase other belief systems. He could not wrap his head around the fact that anyone could abandon Christ and pursue something else.
Once you have had a taste of grace, once you have digested it and learned to live off of its power, you can never ignore and walk away from the God that gives grace. Grace is "unmerited favor". Unfortunately in our culture the unmerited favor of God often gives way to the instant gratification of self. You can't earn grace; it is a gift. A gift from God. And because of that gift you have the reassurance that everything that happens in your life will somehow be utilized in making you stronger, more loving and more faithful. You needed a Savior. God's plan was to give you one. To Him be the glory forever and ever! That's His plan. What's your plan?