There are many in the church who exclude seekers outside of their personal denominations. God came to set the captives free and bind the broken hearted. The Word was made flesh and is The Truth. When we allow the spirit of religion to make Jesus Christ a "members only" club we are following the enemy and not Christ's example. He sought out the lost (in and out of the church) and taught them the Truth regardless of their current belief system. We limit our God and His love for His people when we call anyone who teaches to those who are lost (in and out of the church) false teachers.
We must be discerning but not blinded by the enemy. We must listen to the Spirit's prompting and obey Him even when it is to teach or (oh my word perform a miracle/healing etc) to a or on a "heathen". Christ's love is not exclusive. He died and was resurrected that NONE should perish. Our bible studies and churches should be clear in their teaching of the Gospel but be teaching it to ALL people. The Word will not go void. If a teacher is teaching His Word and The Gospel as the only way to the One True God then The WORD and the HOLY SPIRIT will do the work and move those who have been misled or misinformed and/or blinded into TRUTH.
So often we operate out of fear that somehow these outsiders will infect our churches and we will be overtaken and destroyed. Our God is bigger than that and we do NOT have to operate out of fear. Christ wasn't scared (even as a child) to walk into the temple and teach. He didn't tell those seeking truth that because they were misinformed and lied to that they were not welcome to sit at His feet.
Yes- we must be pressing into the Holy Spirit for guidance and direction. Yes- we must guard against false teachers and wolves in sheep's clothing but, we must do so with His heart and His love for His people not out of the spirit of fear and religion. Those spirits would have us believe that God isn't wise enough or strong enough to move those hearts without us actively pointing out their sin and making them "see" the light first before inviting them into our house. Those spirits cause disunity.
We have been given a trust to guard and He will guard it through us as we do our job. If we are armed with His Word (sword), pray without ceasing for wisdom, look for His fruit and consistency of His character in the teaching, be mindful that we fight against powers and principalities not flesh and blood and hold fast to the Truth that He is fully capable of revealing Himself to ALL people; we can rest in His perfect love and embrace the fact that He will not fail us or lead us astray.
"Beyond all these things put on love, which is the perfect bond of unity. Let the peace of Christ rule in your hearts, to which indeed you were called in one body; and be thankful. Let the word of Christ richly dwell within you, with all wisdom teaching and admonishing one another with psalms and hymns and spiritual songs, singing with thankfulness in your hearts to God. Whatever you do in word or deed, do all in the name of the Lord Jesus, giving thanks through Him to God the Father." Colossians 3:14-17