"Jesus had limited what He told the disciples because of their inability to understand"
"In 14:26, Jesus says, "But the Paraclete, the Holy Spirit, whom the Father will send in my name, will teach you all things and will remind you of all that I myself spoke to you.” (14:26). The teaching ministry of the Paraclete is presented as superior to that of Jesus because the Paraclete will teach them "All things" and will remind them of "everything" that Jesus said (14:26). The Paraclete's ministry is set clearly in the future. Jesus again identifies this "Paraclete" with the Holy Spirit, whom the disciples would know from the Old Testament (Ps. 51:11; Is. 63:10,11): "But the Paraclete, the Holy Spirit—the Father will send in My name" (14:26). This sending will fulfill Old Testament promises as well as Jesus' request (14:16). The sending of the Paraclete in Jesus' name not only links the sending to Jesus' request but also supports the claim that the Paraclete comes to continue Jesus' ministry.
The Paraclete's ministry to the disciples both goes beyond and is limited by Jesus' ministry. On the one hand, the Paraclete would teach the disciples "all things" (14:26), and so the disciples would know more from the Paraclete's teaching than they knew from Jesus' teaching (see 1 John 2:27)... The Spirit's teaching went beyond what Jesus taught only in that it deepened their understanding of what He said.
Jesus' statements in John 14:25-26 are closely paralleled by those in 16:12-15... What may be implicit in John 14:25-26, that Jesus had limited what He told the disciples because of their inability to understand, is made explicit in 16:12. They had trouble understanding what He did tell them, and now the reason for that is made plain: their abilities are limited. Their limitation, however, is not the world's inability to "know" or "see" the Spirit (14:17), but is a function of their location on the salvation-historical time line (note 16:12," you cannot bear them now [arti]" and "Whenever [hotan] the Spirit of truth comes" in 16:13).
As in 14:25-26 the Paraclete will teach more than Jesus taught; in 16:12-13 ... will "guide them into all truth" and speak of "things that are to come," including things they are unable to "Bear.”...
The Spirit will also declare to the disciples "What is coming" (ta erchomena, 16:13). The Spirit's proclamation should be understood in light of the rest of the New Testament. As Holwerda writes,
The task of the Spirit to teach all things, to lead into all the truth, and to declare the things to come is essentially one: the Spirit reveals the meaning of the Heilsgeschichte, the meaning of the saving events, past, present, and future. The Spirit reveals to the disciples the meaning of the work of the historical Jesus, the exalted Jesus, and the Jesus who is to come. The proper commentary on this work of the Spirit is the New Testament itself. [117]
James M. Hamilton, Jr., God's Indwelling Presence: The Holy Spirit in the Old and New Testaments
B&H Academic (August 1, 2006), pp. 79-83
[117] Holwerda, The Holy Spirit and Eschatology in the Gospel of John, 62. Thus Holwerda takes the Spirit's declaration of "What is coming" to refer to: (1) the book of Revelation ; (2) the meaning of salvation history; (3) the meaning of the work of the historical Jesus; (4) the exalted Jesus; (5) the coming Jesus; and (6) the NT.