Why did Jesus Christ instruct us to love from a purity of heart and a good conscience and a sincere faith?  

Why would that be a goal?  

Because our salvation depends upon loving from a purity of heart and a good conscience and a sincere faith.

Without it, we cannot receive salvation, see the Lord and enter the kingdom of heaven. 

"Blessed are the pure in heart,

    for they will see God."

                          - Matthew 5:8

“Truly I say to you, 

unless you are 

converted and become like children, you 

will not enter the kingdom of heaven."

                               - Matthew 18:3

"He who has clean hands and a pure heart,

    who does not lift up his soul to what is false

    and does not swear deceitfully.

He will receive blessing from the Lord

    and righteousness from the God of his 

salvation."

                                   - Psalm 24: 4-5 

What happens when we fail to do as the Lord commands?  

When we fail to love the Lord and love thy neighbor as self from a purity of heart and good conscience and sincere faith?