3 And by this we know that we have come to know him, if we keep his commandments. 4 Whoever says “I know him” but does not keep his commandments is a liar, and the truth is not in him, 5 but whoever keeps his word, in him truly the love of God is perfected. By this we may know that we are in him: 6 whoever says he abides in him ought to walk in the same way in which he walked.
7 Beloved, I am writing you no new commandment, but an old commandment that you had from the beginning. The old commandment is the word that you have heard. 8 At the same time, it is a new commandment that I am writing to you, which is true in him and in you, because[a] the darkness is passing away and the true light is already shining. 9 Whoever says he is in the light and hates his brother is still in darkness. 10 Whoever loves his brother abides in the light, and in him[b]there is no cause for stumbling. 11 But whoever hates his brother is in the darkness and walks in the darkness, and does not know where he is going, because the darkness has blinded his eyes.
1 John 2:3-11, ESV
John starts chapter two, in verse three, by challenging the reader to self-evaluation. He is essentially saying if our claim of knowing Christ is authentic, it will be displayed in the new life of obedience to God. The lack of moral living makes our claim bogus and God a liar.
John further disputes a way of thinking commonly accepted by Gnostics. They believed that when Jesus came to earth, He didn’t possess a body like our own; instead, the Gnostics taught that He only seemed to have a physical body (known as the heresy of “docetism,” from the Greek verb “to seem”). Therefore, leading them to permit their physical passions to run whatever course they choose. They further justified their libertine lifestyles with the erroneous notion that their evil bodies were destined for destruction anyway, while their spirits, which they believed were good, would remain unharmed.
John teaches a greater harmony between our spirit and our bodies than Gnostics. He states that we cannot say we know Him if “our bodies” do not keep His commandments. Failure to do so makes our declaration of belief in Christ a lie.