Pentecost
Quakers don't celebrate lots of feasts and saints' days, but today is Pentecost on many western Christian calendars, celebrating an event in the New Testament which gives tremendous inspiration to the notion that the Spirit can speak through humans:
And when the day of Pentecost was fully come, they were all with one accord in one place. And suddenly there came a sound from heaven as of a mighty rushing wind, and it filled all the house where they were sitting. And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance...
And they were all amazed, and were in doubt, saying one to another, What meaneth this? Others mocking said, These men are full of new wine.
But Peter, standing up with the eleven, lifted up his voice, and said unto them, Ye men of Judaea, and all ye that dwell at Jerusalem, be this known unto you, and hearken to my words: For these are not drunken, as ye suppose, seeing it is [but] the third hour of the day. But this is that which was spoken by the prophet Joel; And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams:And on my servants and on my handmaidens I will pour out in those days of my Spirit; and they shall prophesy. (Acts 2:1-4, 12-18)
Quakers don't speak in tongues all listeners hear in their own language, as the apostles did, but this and other passages inspired the early Quakers (as it later did Pentecostals) to see a direct connection between the Spirit and humans, as in this from George Fox:
And therefore all friends, that are come to witness the Holy Ghost and faith, in which the true praying and building is, which gives victory over the world, which is the gift of God, in which you please God, keep your meetings, and being met together, as you are moved, speak: for they spake as moved by the Holy Ghost, and as the spirit gave them utterance: and they prayed with the spirit, and it helped their infirmities, for they could not tell what to pray for as they ought: but he that searcheth the heart, knoweth the mind of the spirit that teacheth to pray. And here comes to be known the birth born of the spirit, which God the Father of spirits hears, which is not of that birth born after the flesh, but is with that persecuted. Works, IV, 123.