Scriptural teaching and earnest effort are very effective. Consider what happened in the case of Saulo in Guatemala.
وَقَدْ أَحْرَزَ سُولُو تَقَدُّمًا رَائِعًا ،‏ حَتَّى إِنَّهُ ٱبْتَدَأَ يُلْقِي ٱلْمَوَاضِيعَ فِي مَدْرَسَةِ ٱلْخِدْمَةِ ٱلثِّيُوقْرَاطِيَّةِ .‏