What a good question! Just how do we know what to say? And when we know, how do we say it?
First consider Christ - He is the Firstborn, He is our chief Example. He is the Author and the Finisher or our salvation. The steps He walked we are to walk in. And He did have something to say on this...
"So Jesus explained, "I tell you the truth, the
Son can do nothing by himself. He does only what he sees the Father doing. Whatever the Father does, the Son also does." John 5:19. "Anyone who doesn't love me will not obey me.
And remember, my words are not my own. What I am telling you is from the Father who sent me." John 14:24. "
I don't speak on my own authority. The Father who sent me has commanded me what to say and how to say it." John 12:49.
And it same with us...we can do nothing - we can speak nothing - on our own (or at least we shouldn't!). Rather we do and say what we see our Lord - Jesus - doing. And the words we say and the verses we use and the things we do are....His! Directed by Him, prompted by Him, DONE in Him! He has sent us and we speak on/in His authority - not our own. Just as it is with Him and the Father so it is with us and Jesus!
If you try to use verses your own way or own your own initiative, or you try to speak wisely on your own, you will fall flat on your face. *SPLAT* And it isn't going to be pretty or yield much fruit...but if your words come God and you motivation and direction is from the Lord, you will find that the right verses to prove that point and that the clarity to say it will come to you. "“When you are brought before synagogues, rulers and authorities, do not worry about how you will defend yourselves or what you will say,
12for the Holy Spirit will teach you at that time what you should say.” Luke 12:11-12. And don't forget what God said to Moses..."Now go! I will be with you as you speak, and I will instruct you in what to say." Exodus 4:12
One last thing....if one lacks Wisdom in anything, just ask (James 1:5) for it! Ask the Lord for wisdom in knowing what you should say and how you should say it - and He will surely tell you! Just be willing to go, just be willing to let God own/use your tongue and "I will teach you what to say."
(And I find it really cool that you are seeking to get better in these things! That willingness will go a long way!)
