God spoke to Cain after he killed Abel.
Genesis 4:7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.
Joshua spoke to Israel before they entered the promised land.
Joshua 24:14 Now therefore fear the LORD, and serve him in sincerity and in truth: and put away the gods which your fathers served on the other side of the flood, and in Egypt; and serve ye the LORD.
We as the creation of God have the privilege of choice.
Many decisions we make everyday based on choice, food, clothing, work, etc.
We have also been given the privilege of choosing to accept Jesus Christ as Lord and Savior.
Many have accepted Him, but countless others have not.
Only God has not given man the privilege to change the outcome of that choice. If we choose Christ, we will spend eternity with Him; if not, then those who reject Him will spend a Christless eternity in hell.
So to after salvation, a Christian may choose to serve Christ or not. But they cannot change the outcome of those choices.
Luke 6:46 And why call ye me, Lord, Lord, and do not the things which I say?