Let's look at this objection from the Scriptures.
And he went out to meet Asa, and said to him: “Hear me, Asa, and all Judah and Benjamin. The with you while you are with Him. If you seek Him, He will be found by you; but if you forsake Him, He will forsake you. 2 Chronicles 15:2 NKJV
Those are some pretty strong words from the prophet. If you forsake Him, He will forsake you. Those who've learned to rightly divide the Word based upon Old Covenant and New Covenant can rightly conclude this was a strong warning given under the Old. Yet many have adopted this same view concerning our salvation in the New Covenant. Many have concluded rightly that we can't lose our salvation, but by golly, we can forfeit it. This idea of walking away, forfeiting our salvation, "leaving it" or however it is labeled stems from passages such as this rooted in people's thinking.
Let's first analyze this passage. In context, it's not so doom and gloom filled as it appears. The king had just come back from a major battle where he was victorious. God, was, in essence, telling him to place his trust in Him and not his strength or own power. Sounds like Grace to me. Trust in God and not performance or self-efforts. Though there is some Grace revealed in this passage, there still exists this stern warning.
Understanding, that this is the Old Covenant, and judgment for sin and rebellion was swift. There was no mediator, no intercessor, and no price paid for sins. Also, the standard of Law was perfection. Yet, even in the midst of Law, which is a Covenant based upon man's ability to stay faithful, a system of do good get good, do bad and get bad, God's love was still clearly seen.
In order to see and understand God's Grace, it is paramount to understand the severity of God. He is a just and righteous God. Yet He is love and Jesus is Grace. I want to reveal to you this great Grace in action. As we know Jesus' birth didn't usher in the New Covenant. Galatians reveals Jesus was born under Law. It was the Finished Work that established the New Covenant.
Now that we've established that Jesus was abiding under the Old Covenant, let's look at an event in the life of Peter. Peter was very zealous and many times he leaped before he looked. Yet, with all his zeal Peter actually forsook our Lord. You can see it in full context here.
Then he began to curse and swear, “I do not know this Man of whom you speak!” Mark 14:71 NKJV
Here we see Peter curse and swear and deny Jesus. He forsook Jesus. Peter, therefore, should be forsaken by God right? This was under the Old. God shouldn't have anything more to do with Peter. Yet what is Jesus reaction to Peter's denial?
And be sure of this: I am with you always, even to the end of the age.” Matthew 28:20b NLT