Revelation 3:8
NKJV - 8 I know your works. See, I have set before you an open door, and no one can shut it; for you have a little strength, have kept My word, and have not denied My name.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Just a layman here but I found that Jesus opened a spiritual door to reach those in places where most don't know about but scripture talks about. It quite literately is the synagogue of satan that you must stand firm with your spiritual armor to preach the gospel to those who are lost across the spiritual. Paul talks about his experiences in his letters (which was preached to every creature which is under heaven; whereof I Paul am made a minister) as well the other apostles.
It all starts with love of God and love of others... church of Philadelphia (brotherly love).
Just realize that God loves all His creatures and died on the cross for all creation.
Mark 16:15
15 And he said unto them, Go ye into all the world, and preach the gospel to every creature
1 Corinthians 16:9
For a great door and effectual is opened to me, and there are many adversaries.
Ephesians 6:12
For we do not wrestle against flesh and blood, but against principalities, against powers, against the rulers of [a]the darkness of this age, against spiritual hosts of wickedness in the heavenly places
Colossians 1:15 - 23
Who is the image of the invisible God, the firstborn of every creature:
16 For by him were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by him, and for him:
17 And he is before all things, and by him all things consist.
18 And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.
19 For it pleased the Father that in him should all fulness dwell;
20 And, having made peace through the blood of his cross, by him to reconcile all things unto himself; by him, I say, whether they be things in earth, or things in heaven.
21 And you, that were sometime alienated and enemies in your mind by wicked works, yet now hath he reconciled
22 In the body of his flesh through death, to present you holy and unblameable and unreproveable in his sight:
23 If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven; whereof I Paul am made a minister;