1 Timothy 1:5 (KJV) Now the end of the commandment is charity out of a pure heart, and of a good conscience, and of faith unfeigned:
When we start having sincere faith in Jesus Christ and the cross (the gospel) and repented from our sins God takes away all guilt and shame and cleans our conscience making it good. The Holy Spirit, the washing of the Word of God, repentance and forgiveness of our sin keeps our conscience good. When being born again God gives us a new heart 

Ezekiel 36:25-27 (KJV)

25 Then will I sprinkle clean water upon you, and ye shall be clean: from all your filthiness, and from all your idols, will I cleanse you.

26 A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh.

27 And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them.

We receive a heart that is pure and the Holy Spirit dwells in us John 14:17 (KJV) Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you.
Therefore we can start to produce the fruits of the Holy Spirit and love from a pure heart fulfilling what the Lord’s commandment.