The texts you’re referring to are Matt 17:20; 21:21; Mark 11:23. Jesus’ point was not an instruction to speak thus to a mountain; rather, he was illustrating the power of faith. He was speaking hyperbolically (as Paul seems to readily acknowledge in 1 Cor 13:2), but the point is still extremely significant. Believers can do amazing things if they do it by faith; conversely, without faith it is impossible to please God. I take it that the most significant thing God permits us to is to participate in the salvation message. When Jesus spoke of believers doing greater works than he did (John 14:12), it is my understanding that he was speaking especially about bringing the lost to Christ. Now I don’t mean by this that we can effect someone’s salvation; rather, I mean that God uses us to be the instrument by which someone believes (cf. John 1:7; Rom 10:14-15). What is truly remarkable about this is that our salvation and our sanctification are described in scripture as the work of God’s strong arm or as a miracle on the level of God raising Christ from the dead (cf, e.g., Exod 6:6; Eph 1:18-20). It is precisely because God raised Christ from the physical dead that we, too, can be raised from the spiritual dead. I can think of no greater miracle than such a transformation!