Sola fide

The power of which we speak is spiritual. It rules in the midst of enemies and is powerful in the midst of oppression. This means nothing else than that "power is made perfect in weakness" (2 Cor. 12:9) and that in all things I can find profit toward salvation (Rom. 8:28), so that the cross and death itself are compelled to serve me and to work together with me for my salvation. This is a splendid privilege and hard to attain, a truly omnipotent power, a spiritual dominion in which there is nothing so good and nothing so evil but that it shall work together for good to me, if only I believe. Yes, since faith alone suffices for salvation, I need nothing except faith exercising the power and dominion of its own liberty. Lo, this is the inestimable power and liberty of Christians.

Not only are we the freest of kings, we are also priests forever, which is far more excellent than being kings, for as priests we are worthy to appear before God to pray for others and to teach one another divine things. These are the functions of priests, and they cannot be granted to any unbeliever.

Thus Christ has made it possible for us, provided we believe in Him, to be not only His brethren, co-heirs, and fellow-kings, but also His fellow-priests. Therefore we may boldly come into the presence of God in the spirit of faith (Heb. 10:19, 22) and cry, "Abba, Father!" pray for one another, and do all things which we see done and foreshadowed in the outer and visible works of priests.

--Martin Luther, On Christian Liberty, 1520