"And hereby we know that we are of the truth, and shall assure our hearts before him. For if our heart condemn us, God is greater than our heart, and knoweth all things. Beloved, if our heart condemn us not, then have we confidence toward God." (I John 3:19-21)
Here Apostle John describes a Christian’s trials, which are quite extraordinary, for matters of the heart are obscure to many. First he says, “if our heart condemn us, God is greater,” and by these words he reveals that a Christian’s heart may at times condemn him. When a vigilant Christian has dreadful temptations, which the devil effects in his flesh, and the devil shoots fiery darts from the flesh into the heart, evil thoughts occur in the mind and wicked lusts and desires are felt in the will, and, finally, self-righteousness rises as a stern and just accuser of the children of God, condemning them in this way: “How can you be a Christian with so much sin? A Christian should be holy and sinless, but you are like the devil himself.” When self-righteousness preaches thus in the reason, it appears to a Christian that his heart is condemning him, although the heart itself cannot condemn, but it is the devil of self-righteousness who condemns the children of God. And this cunning devil, who comes under the guise of truth, transforms himself into an angel of light [II Corinthians 11:14], and thus many of the penitent are deceived because they cannot understand that the one condemning the penitent is the devil. However, now Apostle John says, “If our heart condemn us, God is greater,” and by these words he shows that not a single Christian should believe his own heart, even if it does condemn him, but that instead he must believe God’s gracious promises, which show that Christ has come to save not the righteous but sinners [Matthew 9:13]...
...Rejoice, therefore, and be exceedingly glad, highly ransomed souls, for your reward is great in heaven! Rejoice, elect souls, for your accuser has been cast out of heaven. He no longer has authority to accuse you, for you have an Advocate with the Father, who intercedes on your behalf. Rejoice and be exceedingly glad, children of God, and cry with a loud voice that you have prevailed by the blood of the Lamb and by the word of his testimony. If you struggle in your most precious faith until death, soon you too can sing a hymn of victory with the angels and all redeemed souls. Soon you can sing a new song on Mount Zion and say, “Now is come salvation, and strength, and the kingdom of our God, and the power of his Christ, for the accuser of our brethren is cast down, which accused them before our God day and night. And they overcame him by the blood of the Lamb and by the word of their testimony.” Amen.
-Lars Levi Laestadius 1854 sermon