Here are a couple of quotes from a book I’m reading for a class. The book is entitled New Testament Theology by I. Howard Marshall. I put some thoughts down too.
Quote #1: “Christ is also alive in the members of the congregation, and indeed his presence is the test of whether they are truly believers (2 Cor 13:1-4). Let them test themselves to see whether Christ is in them (2 Cor 13:5). Evidently, then, there is a form of Christian assurance of salvation that is related to self-examination” (p 289).
In short he’s saying that it is possible to know whether or not you’re in Christ. Add to this Jesus’ statements about trees and fruit as well as the heart and what comes out of it and you have a great set of standards by which to judge your faith (and the faith of others).
Quote #2: “Remarkably, briefly the disciples are told that they must from now onward love one another, and this will be their distinguishing mark in the world. The point is, however, repeated in John 15:9-17, where obedience to this command is the condition for remaining in the love of Jesus. The disciples are Jesus’ friends insofar as they obey his commands, including principally this one [to love each other]. This may seem to clash with the spontaneity and undeservedness of the divine love, just as his love for the Father issues in obedience (John 14:31). The point is emphasized by repetition (John 14:15, 21, 23-24)” (p 506).
Here Marshall links being in relationship with Christ is closely to obedience to Christ’s commands. They are indeed inseparable. This is what John says elsewhere:
1 John 2:4-6
The one who says, "I have come to know Him," and does not keep His commandments, is a liar, and the truth is not in him; but whoever keeps His word, in him the love of God has truly been perfected By this we know that we are in Him: the one who says he abides in Him ought himself to walk in the same manner as He walked.
1 John 5:2, 3
By this we know that we love the children of God, when we love God and observe His commandments. For this is the love of God, that we keep His commandments; and His commandments are not burdensome.