As the old saying goes, "You can lead a horse to water, but you can't make him drink." 
Converting unbelievers isn't our job . . . just show them the "water." 
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I planted, Apollos watered, but God gave the growth.  
So neither he who plants nor he who waters is anything, but only God who gives the growth. 
(1 Corinthians 3:6-7 -- ESV)

So faith comes from hearing, and hearing through the word of Christ.
(Romans 10:17 -- ESV)

“I believe that I cannot by my own reason or strength believe in Jesus Christ, my Lord, or come to Him; but the Holy Spirit has called me by the Gospel, enlightened me with His gifts, sanctified and kept me in the true faith.”
(Martin Luther)