‘Look to yourselves, that you do not lose the things which we have wrought, but that you receive a full reward. Whoever goes onward and abides not in the teaching of Christ, has not God. He who abides in the teaching, the same has both the Father and the Son.'
So the warning goes out to ‘watch out'. They are to look to themselves so that they retain the truth and live in it, lest they lose what the faithful preachers of the truth have wrought in them. Not that he feared that they would. He knew that God was at work in them to will and to do of His good pleasure. But he wanted them to ensure that they worked it out with greatest care (Philippians 2:13), for he did not want them to lose their full reward. This is a rare reference for John rarely speaks of the coming reward, as he rarely speaks of the second coming, but compare 1 John 3:3, which demonstrates his emphasis on both. Those who were not faithful as they were, will lose the possibility of likeness with Christ.
‘Whoever goes onward and abides not in the teaching of the Christ, has not God. He who abides in the teaching, the same has both the Father and the Son.' Going on in a general way is not enough, they must abide in the true teaching concerning the Christ, otherwise it will be evidence that they do not have God. That is, it will mean that they do not know the truth about Him and do not have Him abiding in them. For they cannot have the Father without the Son. Those who do abide in His teaching receive and enjoy both Father and Son. It is not possible to have the One without the Other.