When and why did the church decide that the Holy Spirit be received through the Holy Myron instead of simply by the "laying of hands" as the Apostles did?
The Myron is a "consecrated oil" used in two sacraments: Baptism and Chrism (Chrism is also referred to as the Sacrament of Myron, Holy Anointment, or the Sacrament of Confirmation). It is used in the consecration of a Church and its sacred vessels. Chrism is administered immediately after Baptism. The newly baptized infant joins the church and becomes one of its members through Baptism and is strengthened through Chrism, thus having the right to partake of the Holy Communion.
The tradition of using this sacred oil goes back to the Old Testament. In the Holy Book of Exodus 30:22-32, God ordered Moses to prepare an anointing oil of myrrh, cinnamon, cassia, and sweet calamus mixed with pure olive oil. It was used to anoint kings, priests, the Tent of the Tabernacle and its furniture.
Our Fathers the Apostles granted this Sacrament by the laying of the hands after Baptism as we know from the Holy Book of Acts when St. Peter and St. John laid hands on the people of Samaria, who were Baptized in the name of the Lord Jesus Christ, so they received the Holy Spirit (Acts 9:2-6).
The laying of the hands for the dwelling of the Holy Spirit is a specific rite of the Fathers the Apostles and their successors the Bishops, and as the regions of mission increased, consequently the number of believers and those who entered the faith increased. It was not possible for the Apostles to wander throughout all the countries and cities to lay hands on the baptized, so they established anointing by Myron as an alternative for laying hands for the Holy Spirit dwelling.
The first to make the Myron were the Fathers, the Apostles as they kept certain fragrant oils, which were on the Body of the Lord Jesus Christ during His burial, and they added spices brought by the women who prepared them to anoint the Lord, but He had risen. They melted all of this in pure olive oil, prayed on it in the Upper Room and made it a holy ointment to grant the Holy Spirit to the baptized. They decided that their successors the Bishops could renew the making of Myron whenever it was about to end, by the remainder of the original, so that it never finishes in Churches.
When St. Mark came to Alexandria he brought with him some of the Myron oil made by the Fathers, the Apostles. He used it in the Sacrament of Chrism as well as the Patriarchs who succeeded him, very carefully until the era of Pope Athanasius the Apostolic and the Myron was again made. It has continued as such until the present day.
Again, what specifically is the Myron (Chrism)? Myron is a Greek word, which means ointment or fragrant perfume. In the Coptic Orthodox Church it is a mixture of specific oils mixed with spices that were presented when the Lord Jesus Christ was shrouded. What is used today is the same as what was used in the Era of the Apostles. Truly our Church heritage has preserved many blessings that enrich our life and assist to secure our salvation in exactly the same manner as they did more than 2000 years ago.