Love (η αγαπη). The love just mentioned.

That we should walk (ινα περιπατωμεν). Object clause in nominative case in apposition with αγαπη, with ινα and the present active subjunctive of περιπατεω, "that we keep on walking."

The commandment (η εντολη). The one just mentioned with the same construction with ινα as in 1 John 3:23. John changes from the first person plural to the second (ηκουσατε as in 1 John 2:7, περιπατητε) as in 1 John 2:5; 1 John 2:7.

In it (εν αυτη). Either to αληθεια (truth) of verse 2 John 1:4, αγαπη of this verse, or εντολη of this verse. Either makes good sense, probably "in love." With περιπατεω (walk) we have often εν (1 John 1:7; 1 John 1:11, etc.) or κατα (according to) as in Mark 7:5; 1 Corinthians 3:3; 2 Corinthians 10:2, etc.