To lie to the Holy Ghost. The sin is here pointed out. The lie was not to men, but to God. The apostles were moved by the Holy Spirit. These verses show clearly that the sin was hypocrisy and deception in the name of religion. These verses also show that the gifts of goods were purely voluntary.
(PNT)

Thou hast not lied unto men, but unto God: in that lying unto God is so often charged, and no express mention is made of Ananias’s vow, some excuse him of sacrilege, and charge him the more deeply with ambition, covetousness, lying, and hypocrisy, to the apostles, whom he intended to deceive.
(POOLE)
Acts 5:4:
#Ac 5:4| (4) While it remained, was it not your own? And after it was sold, was it not in your own control? Why hast thou put this thing in thy heart? Thou hast not lied to men, but to God. Here Peter brings together the influence of Satan, and the free agency of the tempted, just as he had, in former discourses, the free agency of men, and the purposes of God (see TFG "Ac 3:17" and see TFG "Ac 3:18"). He demands of Ananias, "Why has Satan filled thy heart to lie to the Holy Spirit," and, in the same breath, "Why hast thou put this thing in thy heart?" The existence and agency of the tempter are distinctly recognized, yet it is not Satan, but Ananias who is rebuked; and he is rebuked for doing the very thing that Satan had done, showing that he is as guilty as though Satan had no existence. Indeed, he is rebuked for what Satan had done. The justice of this is manifest from the fact that Satan had no power to fill his heart with evil, without his co-operation. That he had rendered this co-operation, threw the responsibility upon himself. Peter's knowledge of the deception was the result not of human information, but of the insight imparted to him by the Holy Spirit. This is necessary to the significance of the entire incident, as well as to the purport of Peter's own words.
(OCA 68)
(The Fourfold Gospel and Commentary on Acts of Apostles

Revelation
……………. This internal method of revelation reaches its culmination in the New Testament period, which is preeminently the age of the Spirit. What is especially characteristic of this age is revelation through the medium of the written word, what may be called apostolic as distinguished from prophetic inspiration. The revealing Spirit speaks through chosen men as His organs, but through these organs in such a fashion that the most intimate processes of their souls become the instruments by means of which He speaks His mind. Thus, at all events there are brought clearly before us three well-marked modes of revelation, which we may perhaps designate respectively, not with perfect discrimination, it is true, but not misleadingly, (1) external manifestation, (2) internal suggestion, and (3) concursive operation.
Theophany may be taken as the typical form of “external manifestation”; but by its side may be ranged all of those mighty works by which God makes Himself known, including express miracles, no doubt, but along with them every supernatural intervention in the affairs of men, by means of which a better understanding is communicated of what God is or what are His purposes of grace to a sinful race. Under “internal suggestion” may be subsumed all the characteristic phenomena of what is most properly spoken of as “prophecy”: visions and dreams, which, according to a fundamental passage (Num_12:6), constitute the typical forms of prophecy, and with them the whole “prophetic word,” which shares its essential characteristic with visions and dreams, since it comes not by the will of man but from God.
(ISBE: Revelation)

Some distinguish between φανέρωσις as an external manifestation, to the senses, but single and isolated; while ἀποκάλυψις is an inward and abiding disclosure

The Revelation. Apocalypse, or uncovering, so the Greek word means. The curtain of the future is lifted.
(PNT)