Belief in Jesus is the work of the Holy Spirit as in John 6:29
John 6
29 Jesus answered and said unto them, This is the work of God, that ye believe on him whom he hath sent. -kjv
29 Jesus answered and said to them, “This is the work of God, that you believe in Him whom He sent.” -nkjv
29 Jesus answered, “The work of God is this: to believe in the one he has sent.” -niv
29 Jesus replied, “This is the work of God—that you believe in the One He has sent.” -hcsb
In preaching the gospel to non-believers, we need to be sensitive to the work of God, love them and be consistent and faithful ourselves.