It is the Holy Spirit’s ministry in and through the church.
Looking at the lawlessness that is increasing around the world in our day, can you imagine what it would be like when the Church, which through the power of the Holy Spirit within is said to be the light and salt of this world, is removed at the rapture? If things are bad now those days will be absolute chaos. There will be no push-back on the works of the enemy and his evil agenda, and the vast majority will fall into line, thoroughly deceived and wondering in awe at the Antichrist.
Here’s a visual:
The Holy Spirit’s role in the tribulation
We need to be clear that the removal of the restrainer in relation to the coming of the Antichrist does not mean that the Holy Spirit is no longer on the earth. The Holy Spirit, being God, is omnipresent and will always be present on earth. However, his present day restraining work through the Church will be removed at the rapture before the tribulation and the Spirit’s specific role in restraining the man of lawlessness (and the spirit of lawlessness) will be removed.
This does not mean that the Holy Spirit will not carry on other existing roles. He will still save and empower individuals in the tribulation. Clearly He will. All of God’s servants need the power of the Holy Spirit… in every age. Even in the Old Testament the Holy Spirit would come upon individuals to empower them for the task at hand. For example the Holy Spirit came upon Bezalel (Exodus 31:3), Moses (Numbers 11:17, 25-26), Joshua (Numbers 27:18-19), Othniel (Judges 3:9-10), Gideon (Judges 6:34), Epitaph (Judges 11:29), Samson (Judges 13:25; 14:6, 19; 15:14), Samuel (1 Samuel 10:6-10; 16:13-14), Saul (1 Samuel 19:20-23), David (1 Samuel 23:1), Isaiah (Isaiah 59:21) and Ezekiel (Ezekiel 2:2).
So in the tribulation period the Holy Spirit is still operating in His role to save and empower but He is not restraining the man of lawlessness and the spirit of lawlessness. With the restraining hand upon the Antichrist removed, Satan, through the Antichrist, will finally be able to perform all the signs and wonders he desires to deceive the whole world.
2Th 2:9-10 …the one whose coming is in accord with the activity of Satan, with all power and signs and false wonders, and with all the deception of wickedness for those who perish, because they did not receive the love of the truth so as to be saved.
But at the same time, the Holy Spirit will still perform His role of saving and empowering. The 144,000 will be empowered by the Holy Spirit in a radical way as will the 2 witnesses. Through the ministry of the 144,000 countless people will be saved as seen in Revelation 7:9-17. But it will be an unspeakably terrible time and these believers will nearly all lose their lives. This is why, in the book of Revelation, we see the Holy Spirit encouraging the tribulation saints that though they will lose their lives, rest and rewards await!
Rev 14:13 Then I heard a voice from heaven say, “Write: Blessed are the dead who die in the Lord from now on.” “Yes,” says the Spirit, “they will rest from their labor, for their deeds will follow them.”