The aim of our charge is love that issues from a pure heart and a good conscience and a sincere faith. (1 Timothy 1:5 ESV)
If the law must be directed to this object, that we may be instructed in love, which proceeds from faith and a good conscience, it follows, on the other hand, that they who turn the teaching of it into curious questions are wicked expounders of the law. Besides, it is of no great importance whither the word love be regarded in this passage as relating, to both tables of the law, or only to the second table. We are commanded to love God with our whole heart, and our neighbors as ourselves; but when love is spoken of in Scripture, it is more frequently limited to the second part. On the present occasion I should not hesitate to understand by it the love both of God and of our neighbor, if Paul had employed the word love alone; but when he adds, "faith, and a good conscience, and a pure heart," the interpretation which I am now to give will not be at variance with his intention, and will agree well with the scope of the passage. The sum of the law is this, that we may worship God with true faith and a pure conscience, and that we may love one another. Whosoever turns aside from this corrupts the law of God by twisting it to a different purpose.
But here arises a doubt, that Paul appears to prefer "love" to "faith." I reply, they who are of that opinion reason in an excessively childish manner; for, if love is first mentioned, it does not therefore hold the first rank of honor, since Paul shows also that it springs from faith. Now the cause undoubtedly goes before its effect. And if we carefully weigh the whole context, what Paul says is of the same import as if he had said, "The law was given to us for this purpose, that it might instruct us in faith, which is the mother of a good conscience and of love." Thus we must begin with faith, and not with love.
"A pure heart" and "a good conscience" do not greatly differ from each other. Both proceed from faith; for, as to a pure heart, it is said that "God purifieth hearts by faith." (Acts 15:9.) As to a good conscience, Peter declares that it is founded on the resurrection of Christ. (1 Peter 3:21.) From this passage we also learn that there is no true love where there is not fear of God and uprightness of conscience.
Nor is it unworthy of observation that to each of them he adds an epithet; for, as nothing is more common, so nothing is more easy, than to boast of faith and a good conscience. But how few are there who prove by their actions that they are free from all hypocrisy! Especially it is proper to observe the epithet Which he bestows on "faith," when he calls it faith unfeigned; by which he means that the profession of it is insincere, when we do not perceive a good conscience, and when love is not manifested. Now since the salvation of men rests on faith, and since the perfect worship of God rests on faith and a good conscience and love, we need not wonder if Paul makes the sum of the law to consist of them.John Calvin, Commentary on 1 Timothy