You speak of once in grace; always in grace. I have also heard it called once saved; always saved. This is called the doctrine of eternal security. It means that once a person is truly saved, they can never be lost after that. The opposite of the doctrine of eternal security is the belief that a person can by wilful acts of sin (different people have different ideas concerning what point this happens) lose their salvation.
I believe in eternal security (or once in grace; always in grace) for a large number of reasons. However, I also admit that there are a number of doctrinal and practical problems that have to be dealt with. But that is true no matter which way this is taught. But do not think that I take this doctrine lightly. It matters, and it matters very much. If we can lose our salvation, then we must have some part in earning our salvation. If we earn our salvation, then it is not by grace (which means God does for us what we cannot do for ourselves).
Salvation is entirely a gift of God. True, we accept it, but we do not earn it. When we accept it, God births us into His family and we become a child of God (John 1:12). My children were born to me. And, though they have not always been in good graces with me, they have never ceased to be my children. And, though human parents might cruelly cast a child out of their home, the Bible promises us that we can never be separated from the love of Jesus Christ (Romans 8:35-38).
Can I stop being God's child if I want to? Let me ask you: Can you stop being your earthly father's child even if you want to? Of course not--to both questions. Besides, why would anyone want to cease being the child of God? Why would anyone want to go to hell instead of heaven. They would have to had lost their mind to want to stop being saved. I would not cast my children out because they lost their mind; neither would God cast His children out.
Often people will point out to me people they know that "got saved" and later left the Lord and went back to sin. There are two possibilities. One is that they were never saved in the first place. 1 John 2:19 speaks of a group of so-called believers who left the saints so that it might be known that they were never really one of the saints. Many people go through an experience or "get religion" for a time, but they are not saved. Eventually, their lack of true spiritual life bears fruit and they leave the God they never really knew.
The second possibility is that God is still dealing with them, though we may not see it for a time. The Bible teaches that those who are true sons of God will be chastened (whipped) when they stray from Him (Hebrews 12:5-7). In fact, he says that those who do not receive chastisement are not His sons (Hebrews 12:8) but are illegitimate (the Bible uses a stronger word here). Sometimes, the ways of the Lord work slowly over a long period of time, but He is always working to bring His true children back to Him.
But, when we are saved and truly born again, we will forever be one of God's children. We may stray and we may suffer for our disobedience, but we will never cease to be His child. This shows us the greatness of the salvation God has given us. Thanks be to His name.