Paul the apostle of grace said in 1 Corinthians 9:27.
“But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.”
Jesus said to the church at Philadelphia in Revelation 3 v 11
Behold, I come quickly: hold that fast which thou hast, that no man take thy crown.
When you are given a scholarship, there is an acceptable CGPA in order to maintain the scholarship. There is an acceptable standard for remaining in God’s kingdom. The prodigal son is a classic example of someone that strayed away, and although the father is ever ready to receive him, he could never have returned if he didnt get a point of realisation and change of heart.
We are saved by grace and adopted into God’s kingdom. But we have a part to play so that we do not loose our position in God’s family. We need to be watchful and prayerful… “watch and pray that ye enter not into temptation, the spirit is willing but the flesh is weak.”
Careless believers can loose their salvation but there is always an opportunity to return, but many stray away and never return.
2Peter 2 v 20 – 22
 For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning.  For it had been better for them not to have known the way of righteousness, than, after they have known it , to turn from the holy commandment delivered unto them.  But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire.
There is eternal salvation subject to consistent Christian living but there is no eternal security. A careless Christian can lose his salvation and be entangled in sin.