The thesis statement is an absolutely critical aspect of any well-structured speech. The thesis should be one stand-alone sentence which tells the audience your main line of logic, your argument, what you are going to talk about, etc - it cannot rely on any previous sentences. Think of it this way: you are on an elevator with a person who is interested in your perspective, but you only have 15 seconds to convince him of your position, since the elevator will be at the top and both of you will have to leave. So, the thesis statement is kind of like your "elevator speech."
For Apologetics in particular, having a good thesis reinforces your structure and tells the judge where you are going, but not in the exact same way as a "roadmap," where you give your two or three point outline before you dive in to your speech. I particularly agree with what Maggie said earlier:
The thesis statement is basically the answer to the question or the significance of the answer. Your thesis statement should come up in different forms at different times and be the main message or point of the speech.
For the card, Define and Defend the Righteousness of God, I would state my thesis by saying: "God's righteousness creates a wall between the Lord and sinners that can only be broken down through Christ"
Alluding to the thesis throughout your speech is important. If you say something in your thesis, it better be discussed in your speech. Likewise, if you say something in your speech, it better be incorporated into your thesis. Otherwise, there is a disconnect between the thesis and your speech. Writing coherent theses is challenging at first, but once you get the hang of it writing a thesis will become second nature to you and will be very helpful in many areas of life, not just in Stoa competition.
Hope that helps, Rachael!