The word "united" ("planted" in the King James) in verse 5 draws our attention because it is elsewhere translated "grafted" or "engrafted." In John 15, Christ describes Himself as a vine, and we are its branches. In Romans 9, Paul compares converted Israelites to natural branches and Gentiles as unnatural branches grafted into the same vine. Union is achieved with all sharing a part. All are receiving of the same source, and all are striving to produce the same fruit.
But how do we know we are attached to that Vine?
Now by this we know that we know Him, if we keep His commandments. He who says, "I know Him," and does not keep His commandments, is a liar, and the truth is not in him. But whoever keeps His word, truly the love of God is perfected in him. By this we know that we are in Him. (I John 2:3-5)
There must be something that proves we are united with the Father and the Son, engrafted as part of Them and in union with Them. That something is the manner in which we conduct our life.
Language identifies people and so does the clothing they wear. A person's name is probably the most common of all identifiers. But the sons of God, those in union with the Father and the Son, are identified by commandment-keeping. It verifies that we are united with Christ. Commandment-keeping is love. Biblical love is an action, not merely a feeling. It may contain a positive feeling, even outright affection and passion for the one or ones who are the recipients of the act of love, but its foundation lies in the act rather than the emotion.
Acts of love without emotion can be entirely sterile. This extreme is not biblical love. At the other extreme are those who emotionally say they love Christ. What they say is probably true - as far as their understanding of love goes. Their declaration of love for Him may be motivated by feeling awe and gratitude springing from a recognition that He is indeed Creator, Savior, and High Priest, and that this awesome Being actually humbly sacrificed His life for them personally. Often, such people will then proceed to break His commandments, proving they do not know what love is.
In that kind of relationship, feelings eventually run dry, and the relationship and therefore the union ends. The love of the Bible is always first moral. This morality verifies we are yielding to Him. John commands us in I John 2:6 to walk as Jesus walked, and Jesus walked morally. The only way we can be conformed to the image of Jesus Christ is to walk as He walked.
This is the reason for our standing with God. We stand before Him as Jesus Christ for the very purpose of living life as He did as closely as possible. We cannot say we do this perfectly because our actions and reactions, our tempers and feelings, our sins of omission and commission betray us, revealing our continual need for the application of Christ's blood to restore our standing to the pristine standard of our Savior, even if for only a short time. Our gratitude to God for His thoughtful foresight and merciful patience is thus renewed in the acknowledgement of our sin.
— John W. Ritenbaugh