Paul told those in Corinth that he was concerned that they would be moved away from the simplicity of the Gospel. The gospel message is spelled out in 1 Corinthians 15. The message says that Jesus died for our sins according to the scriptures, and three days later rose again bodily and was seen by many witnesses.
Furthermore, we who believe this message will eventually be changed when He comes – our corruptible bodies will be clothed with incorruptible that we may inherit the Kingdom of God. The Gospel message concerns His coming to die for our sins so that we can be forgiven and reconciled to God and His second coming specifically for those who believe in Him to be raised up into immortality and everlasting Life. The rest of the New Testament is spent clarifying and unfolding the details and implications of these two great events – His first coming for sin, and His second coming unto salvation.
The message is very simple, and its requirement is very simple: to simply believe it. This simple message is the very power of God unto salvation to those who believe. It is the vehicle through which God works everything for those who believe it. Receiving this simple message, understanding it, clinging to it, and increasing in confidence in it is the growth in our salvation.
The most important – perhaps the only important thing for us as Christians to do is to pay heed to this message, to continually clarify and remind ourselves and others of it. As we do so, we grow in faith and increasing assurance about our stand before God.
Heb 6:11 And we desire that every one of you do shew the same diligence to the full assurance of hope unto the end: That ye be not slothful, but followers of them who through faith and patience inherit the promises
We should grow to become fully assured so that our heart is full of thankfulness, and we can boldly stand before Him at His coming. This is what it means to abide in the Lord and in His Word. We simply stay focused on the Gospel message.
Our entire Christian life is to be characterized by a vigilance to guard something we have received. We are to remain focused on it until we really know that we have possessed it. Jesus said:
Luk 8:18 Take heed therefore how ye hear: for whosoever hath, to him shall be given; and whosoever hath not, from him shall be taken even that which he seemeth to have.
This corresponds with the warning in Hebrews not to neglect the great salvation that was preached to us:
Heb_2:3 How shall we escape, if we neglect so great salvation; which at the first began to be spoken by the Lord, and was confirmed unto us by them that heard him;