Exegesis is what Jesus does as the one who gives a perfect exposition of God. (John 1:18 NASB) "No man has seen God at any time; the only begotten God, who is in the bosom of the Father, He has explained Him." So Jesus could say, "Whoever has seen me has seen the Father." (John 14:9)
Eisegesis is another thing altogether. You can see the parallel between eisegesis and exegesis. The difference is the prefix. Instead of the "Ex" from the Greek word for "Out of," this word has "Eis" from the Greek word for "Into."
The difference in meaning is significant. Instead of carrying the meaning out of the text and unpacking it in order (as in exegesis), eisegesis means you carry meaning into the text and create a new meaning that was not there.
It has been said that "you can prove anything you want from the Bible." Well, that is not true if you practice careful exegesis. In careful exegesis you come up with the meaning that is already there through observing the words in their normal relationships and given their normal definitions and taking into account the bigger picture of the history and context and type of material that is being studied.
But in eisegesis it is true that you can prove anything. In eisegesis the meaning doesn't really come from the text at all - it comes from the mind of the person who is doing the eisegesis. That person is reading his or her thoughts and prejudices into the text. Why should we be surprised that they find their unique doctrine everywhere they look? One of the telltale indicators of eisegesis is that the interpreter has no problem teaching his point no matter what text he is looking at. He cannot see a contrary point anywhere - for him everything fits together beautifully!
Exegesis is hard work, partly because of the need to be careful to avoid eisegesis. As a preacher there is always a temptation to approach a sermon with a desire to make the text support what I want to say. But the job of expositional preaching is to find out what the text is saying and to preach THAT.
(2 Timothy 4:2–4 ESV) preach the word; be ready in season and out of season; reprove, rebuke, and exhort, with complete patience and teaching.
3 For the time is coming when people will not endure sound teaching, but having itching ears they will accumulate for themselves teachers to suit their own passions,
4 and will turn away from listening to the truth and wander off into myths.