Summary: God has designed His born again people so that they may be empowered for glory & service. The church's glory will not be acquired by human might & power but only by the Spirit of the Lord
Power means force just as might means force. But not in a collective or numerical sense such as army. Power is the ability to persist purposefully, a resoluteness, resolve and consistency.
‘Might' refers then to material wealth & military capability, while ‘power' refers to the resolve of it. Together they represent the full extent of human resources which could be deployed to deal with a task, be it by an individual or the combined life effort of a multitude. Yet all man can do in and of himself is unable to accomplish God's task. Not by anything man can do, can man do anything for God (Jn. 15:5). A greater agency must be secured. God ends the hope that true success, success of eternal value, can be achieved by anything other than God. How is God's work to be accomplished then? It takes the supernatural or the divine factor to accomplish the work of God.
God's work must be by God's Spirit (ruach). It is by the supernatural enabling of God alone that men can accomplish the work of the Lord in the world. God's Spirit is the only absolutely necessary resource on which we must depend. [Read 2 Cor. 10:3-5.] From first to last God's work is a spiritual work. It is only through God's Spirit that anything of eternal value is accomplished. The power needed for spiritual victory is of God and not of man. God's work is accomplished by the invisible, intangible, unchangeable, but present, potent, and personal Spirit of God.
There are three ways we can attempt to do the work of God. We can trust our own strength and wisdom; we can use the same resources the world uses; or we can depend on the power of God. The first two approaches may appear to succeed, but they'll fail in the end. Only work done through the power of the Spirit will glorify God and endure the fires of His judgment (1 Cor. 3:12-15).
The secret to shining forth the light of God is that all power and sufficiency are of the Holy Spirit. Notice that the Holy Spirit is illustrated as the oil which flows through the lampstand and the lampstand is representative of the Church. The oil flowed through appointed conduits into the bowl and through fixed channels connected to the main stem to the individual flames. (Eg. fixed channels outlined in the word of God - repentance, life of obedient faith, etc). It is through fixed channels that the Spirit of God will flow to light up the church of God and its individual members.
What is being said "not by might nor by power but by My Spirit" is that the strength and power of the church or of individual believers is found solely in the Holy Spirit. Where lies the power of the church, in numbers? God would rather have Gideon's Army of 300 consecrated souls than 20,000 who are living according to the course of this world. Where lies the power of the church, in human wealth or political strength? Sometimes these are curses instead of blessings. The power of any church is the Holy Spirit. If He is flowing so that Christ shines forth in a pastor and in a congregation of disciples, then there is power to do an eternal work for God!