What this commandment is which John mentions, and whether it refers to a specific commandment or in general to the directives contained in the Word of God, is difficult to say.  It could be thought to refer to Christ’s new commandment, but I find this doubtful since John will introduce that in the following verse, and he almost certainly would have called it a command of Christ, rather than of the Father.  I suspect it would be better to presume the commandment from the Father to be that which is issued in general to all creation, to believe on the Son of God.  This is the very first act of the obedience of faith, the very first step on the road to heaven.  We can never walk in the truth, except we embrace that first and greatest of truths, that Christ Jesus came into the world to save sinners.  This would be an entirely acceptable interpretation of the commandment received from the Father; but I rather prefer to give it a more general application, and refer it to the entire framework of God’s commandments, which embrace faith, love, and all the duties attendant upon these first graces.  John uses the indefinite article a, rather than the definite the, which seems to lend credence to my argument.  Had it been a specific commandment he was referencing, it stands to reason he would have used the definite article.  Therefore, we may refer the words back to the phrase “walking in the truth,” and conclude that the commandment received from the Father embraces every law and ordinance of God which a believer must follow, if he would remain steadfast in the truth of his Lord.