There are questions about ourselves that are easily answered, and there are other questions that present more of a challenge. If someone asks me, “Are you a husband?” I can show them my ring, present my wedding certificate, point to the woman standing next to me who shares my life and my last name. Yes, I am 100% sure that I’m married.
If someone asks me, “Are you an employee?” I can show them where I work, present my pay stubs, point to the truck with which I make deliveries. Yes, I am 100% sure that I’m an employee.
Other questions are not so easily answered. If I’m asked, “Are you a good husband?” what immediately comes to mind are the times I’ve failed my wife, acted selfishly, and been anything but a good husband. I have no real external, tangible, objective way to answer that question. I must rely on feelings and speculations. Similarly, if someone asks, “What kind of employee are you?” my mind goes to the labor I’ve put in, but also to the times I’ve slacked off yet expected a full paycheck for a half-hearted performance. What if I think I’m doing an okay job but my boss thinks differently and fires me?
There are questions about ourselves that are easily answered, and there are other questions about ourselves where we have to explore our hearts to test their sincerity, take account of the good and bad things we’ve done, focus inwardly to find the answer.
We don't look within ourselves to find out if we are Christians, but within Christ.
What about the question, “Are you a Christian?” Does this one belong to that second category, where we must explore our hearts, test our actions, focus inside ourselves to get to the right answer?
That’s certainly what some people think. So they urge folks to ask themselves if they really believe, if they really love their neighbor, if they really live a moral life. But no matter how well intentioned such an urging might be, rather than helping, it pours the poison of doubt into the souls of those for whom Christ died.
Look inside yourself to answer, “Are you a Christian?” and what will you find? You'll find a heart that is deceitful above all things (Jer 17:9); a heart from which flow evil thoughts, murder, adultery, sexual immorality, theft, false witness, slander (Matt 15:19); a conscience that testifies that nothing good dwells in you, that the evil you do not want to do, you nevertheless keep right on doing (Rom 7:18-19).
Look at your deeds to answer, “Are you a Christian?” and what will you find? You'll find that all your righteousness is as filthy rags (Isa 64:6). And if such be your righteousness, how dirty and defiled must be your unrighteousness. Look at your deeds and you will find that even when you have the desire to do what is right, you don’t have the ability to carry it out (Rom 7:18). Even if you did all that you were commanded, you must still say, “I am an unworthy servant; I have only done what was my duty,” (cf. Luke 17:10). If such be the response of a person whose has kept all God’s commands, then we who have broken those commands are worthy of nothing but punishment, now and forever.
Thus, to answer, “Are you a Christian?” by looking inside ourselves, or by looking to our deeds or love of the neighbor, is to drink the poison of doubt. The more Christians look at themselves to see whether they are Christians, the more they will become convinced that they are not Christians.
We don't look within ourselves to find out if we are Christians, but within Christ. Our assurance is in his objective, external work of salvation on our behalf. Not in our hearts but in the heart and life and death and resurrection of Jesus Christ do we receive assurance that we are the children of God.
It is not our faithfulness that saves us, but the faithfulness of Jesus.
In Christ, God was reconciling the world to himself (2 Cor 5:19)—the world of which you are a part. In Christ you are reconciled to God, at peace with the Lord, adopted as a child of the heavenly Father. God loved the world in this way: by sending his only begotten Son to die as the Lamb of God who takes away the sin of the world. And if the world’s sin is taken away, then your sins are taken away. God made him who knew no sin to be sin for us in order that in him we might become the righteousness of God (2 Cor 5:21). His worthiness covers our unworthiness.
Your name is written in the wounds of Jesus. He has dipped his pen in the crimson ink of his veins and written your name, indelibly, in the Lamb’s Book of Life. He has engraved your name on the palms of his hands. He has tattooed his name onto your soul and heart and mind and body—you are completely and everlastingly his and his alone. In baptism you did not commit yourself to Christ; he committed himself to you. More than that, in those waters he crucified you with himself, laid your body with his in the tomb, and he carried you forth into the light of life again. He who believes and is baptized shall be saved. That believing, that faith, is not a conviction you created but a gift you received. By the Holy Spirit you confess, “Jesus is Lord.”
Do we still struggle to believe? Of course we do, for we are far from perfect in this life. As a father once prayed to Jesus, so we also pray, “Lord, I believe; help Thou my unbelief,” (Mark 9:24). And he does. He enlivens and strengthens our faith by continuing to forgive us, to love us, to heal us, to give us himself. It is not our faithfulness that saves us, but the faithfulness of Jesus. For even if we are faithless he remains faithful, for he cannot deny himself (2 Tim 2:13).
How do you know you’re a Christian? Not because your heart is good and pure but because the heart of Christ pulses with a love for you that will never end. Not because your deeds are righteous but because he has been righteous on your behalf and clothes you with that righteousness. Not because you have lived for him but because he has lived and died and risen again for you. Not because you asked him to be your Savior but because while you were yet a sinner, Christ died for you, chose you, called you, and washed you clean in his own divine blood.
If someone asks you, “How do you know you’re a Christian?” the answer is as simple as it is beautiful: you know you’re a Christian
...because Christ has made you his own
...because Christ will hold you fast
...because nothing can separate you from the love of God
...because Christ knows you, forgives you, baptizes you, and will never let you go.
That’s how you know you’re a Christian.