John 16:12 - Albert Barnes' Notes on the Bible

I have yet many things to say ... - There were many things pertaining to the work of the Spirit and the establishment of religion which might be said. Jesus had given them the outline; he had presented to them the great doctrines of the system, but he had not gone into details. These were things which they could not then bear. They were still full of Jewish prejudices, and were not prepared for a full development of his plans. He probably refers here to the great change which were to take place in the Jewish system - the abolition of sacrifices and the priesthood, the change of the Sabbath, the rejection of the Jewish nation, etc. For these doctrines they were not prepared, but they would in due time be taught them by the Holy Spirit.