@oristarA,
This is religious-speak, which is slightly different than normal English.
To be in control in this context means to control your own life. If you are in control that you are making your own decisions and choosing what you want to do for yourself. The writer suggests that it should be God controlling your life (and making decisions for you).
The phrase "be in control" can be used in non-religious contexts (with a slightly different meaning).
A child who is "out of control" is usually having a temper tantrum or is doing something really bad that you can't stop.
If you are "in control" when you are driving that you are perfectly able to steer the car. If you are "out of control" then you are skidding or going to fast. You can then "gain control" of the car again.
At my work, my boss was fired and there was a struggle for who had the power (authority). My new boss came and said "I am in control now" meaning that he now had the power and authority and that we had better do what he said.