In Psalm 136, this phrase is repeated in every verse, which is a total of 26 times:
For His lovingkindness is everlasting. (NASB)
for his steadfast love endures forever (ESV)
Have you ever seen in movies, or worse, have you ever known anyone personally who comes to a point where they tell their spouse that they don't love them anymore? What is it that makes one's love for another fade? Perhaps some of these don't even have in mind the right definition of love. They have given other emotional or physical feelings the title of love. Whatever the reason, we humans are sinners and this act of "falling out of love" occurs often.
But look at this phrase found in every verse of this Psalm. God's love endures forever, it is everlasting, it has no end! No matter how we fail Him in our sin, if we are true believers, His love for us continues. We can do nothing that would cause Him to cease loving us. That is a glorious promise and truth of the Bible. This is not a licence to sin, but a great comfort during those times when we truly have blown it and acted in unbelief and disobedience. We are secure in God's love.
Luther in a letter to Melanchthon wrote the following:
If you are a preacher of mercy, do not preach an imaginary but
the true mercy. If the mercy is true, you must therefore bear the true, not an imaginary sin. God does not save those who are only imaginary sinners. Be a sinner, and let your sins be strong, but let your trust in Christ be stronger, and rejoice in Christ who is the victor over sin, death, and the world. We will commit sins while we are here, for this life is not a place where justice resides. We, however, says Peter (2 Peter 3:13) are looking forward to a new heaven and a new earth where justice will reign. It suffices that through God's glory we have recognized the Lamb who takes away the sin of the world. No sin can separate us from Him, even if we were to kill or commit adultery thousands of times each day. Do you think such an exalted Lamb paid merely a small price with a meager sacrifice for our sins? Pray hard for you are quite a sinner.
Luther's comments about "let your sins be strong" has given some difficulty in this letter. I thing Luther is just saying that Melanchthon should be strong to confess his sin before the Lord. Many play games with words and are not bold in calling sin what it is. Only those who quit relying on their righteousness and trust in Christ's righteousness and substitutionary death can know that they are secure in God's mercy and love. A love that is everlasting. So be bold in admitting your sin so that your trust in Christ's work for you will be strong. No sin can separate us from His love. Even as the apostle Paul reminds us in Romans 8:38-39:
For I am convinced that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers, nor height, nor depth, nor any other created thing, will be able to separate us from the love of God, which is in Christ Jesus our Lord. (NASB)