Summary: You can be speaking clerly and saying nothing unless you speak to the audience.
Once you know the message and the entreater you must know the audience. Those who are in evangelism and revival ministries understand this principle. When they stand up to speak to an audience they have never met before they have to make some basic choices based on the the pastor, if they know him, the doctrinal position of the church though that can still be tough as many church people today know little about doctrine and with six Baptists you can get thirteen opinions. Check the physical makeup of the church. Are they primarily young, old, well mixed, one culture, multi-cultured, etc. Are you in the North, South, East or West. All of these factors will determine what direction you need to take in your sermon, applicable stories or analogies if you are going to make any impact at all.
Many colloquialisms and historical references that would work well in Texas would be curiosities or confusions in Maine. Sometimes I will not realize that I am using terms or acronyms that are quite common at my job or my church but are only vaguely familiar or unknown to the people I am talking to at the time. When I see that glazed look or they say they have no clue as to what I just said that I know I have to back up and use a different approach. Having served in three military branches as well as working for the Veterans Administration I could use a great many acronyms that would confuse Socrates. or Einstein. People might think me quite intelligent, but there has been no real communication.
Keep It Simple, Saint is not enough because what is simple to you may not be to the group you are trying to convert or teach. I work in the computer field and I try very hard to keep the cookies on the bottom shelf so everyone can eat them when I try to explain things to my clients. Still even using simple language and down home humor and illustrations some folks still do not get it and just acquiesce to let me do what I need to do. Their minds are just not wired to understand that particular set of information.
2 Cor 4:3-4
3 But if our gospel be hid, it is hid to them that are lost:
4 In whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, who is the image of God, should shine unto them. KJV
7 What then? Israel hath not obtained that which he seeketh for; but the election hath obtained it, and the rest were blinded
8 (According as it is written, God hath given them the spirit of slumber, eyes that they should not see, and ears that they should not hear;) unto this day.
9 And David saith, Let their table be made a snare, and a trap, and a stumblingblock, and a recompence unto them:
10 Let their eyes be darkened, that they may not see, and bow down their back alway.
You are going to run into people who are not going to get the Gospel because of satanic blindness or because of the hardness of their heart, self-inflicted or God caused, as we see in these passages. No one is saved by clever speeches or being defeated in a debate. You may have vast intellectual knowledge of archeology and various science disciplines, excelled in debate club in college and read your Bible in the Koine Greek on Mondays, Wednesdays and Fridays, but in Aramaic on Tuesdays and Thursdays saving the Saturdays and Sundays for Hebrew, but there are people who will not be impressed. They are not ready to be saved or they are the ones that never will be saved. Accept that and pray that God will send someone to them when they are ready or weep for the fact that they are dead men walking.
When Peter gave his sermon on Pentecost and won three thousand souls he spoke to them from the background of Jews and tied them to current events of Jesus' crucifixion and resurrection. He did not speak to Greek poetry, politics or economic issues. He spoke to the reason for what the audience was seeing repudiating their accusation of drunkenness and taking them to the Scriptures speaking to it and then explaining how Christ caused it to happen then pointing to their guilt. As one man said, start with your text and make a beeline to the Cross. The result was conviction, repentance and salvation. He used the right material to the right crowd and yet we know that sheer man's wisdom was not all involved in this.
There was a time in this country when you could preach to unbelievers and they understood what you were saying because they had been exposed to the Scripture as a child at home, school and even church. They may have been religious like the Jews in Acts or scoffers, but they understood because they heard the Bible in many places and scenarios so they were not ignorant. Many could have quoted Scripture for you. I have had many an interesting conversation with a person who grew up as a preacher's kid or overheard some deep and even sound theology discussed by drunks. In a session of prison ministry I have heard men read poems or sing songs they wrote and even had one fellow break out in a sermon when we were somehow forgotten by the guard and were there three hours instead of one.