How should I react in this situation?

Look, the bishop of my church has asked me to play the bass guitar in the church musicians group. I told him that I didn't know how but he was still enthusiastic about having me play. He said that it doesn't matter and about a month or two later the bishop's son bought me a bass guitar so that I could learn. About Once every week I would get together with the other musicians (the pianist and the drummers) and play during the church service.Well, I'm not a professional, eveytime I would play, the bishop's son, who is an experienced bass player, would show me a different trick or way to play the bass. So it's been about 3 weeks since they bought the bass guitar for me and when I wasn't playing in the church, I would be learning at home at my own pace. Last Sunday a group of my friends and I were talking about the music at the church(the drummer was in the group). One of my friends started talking about how he knew how to play the bass guitar and that he took lessons at his school, so he's experienced. The drummer was surprised that guy knew how to play and he ran to tell his father (the bishop's son). The bishop's son came out and talked to my friend saying "you know how to play the bass? Why didn't you tell me?" he told the both of us to bring our guitars on the next Saturday for rehersal. So my Problem is that I just started playing the bass 3 weeks ago and I'm not starting my guitar lessons for a few more days. This friend of mine is experienced and he has had lessons for probably months. I guess I feel threatened and intimidated because he says that he knows stuff that I still don't know yet. Even though the bishop asked me to play (knowing that I didnt know how), I feel that he and he other musicians may see that my friend knows more and then toss me to the side and focus on him. What if he's really good and they tell me that he will be playing instead of me? I guess I'm just feeling a little insecure. The rehearsal is this Saturday and I'm really nervous, I feel like they'll laugh in my face once the see the difference between me and my friend's playing ability. The other musicians and I haven't even heard him play yet and we're all growing anxious. I'm super nervous, so please tell me. How should I react, and what should I do in a situation like this??