Many people have a last will and testament in order to make clear what they wish to be done with their estate after they pass away. This helps the family avoid difficulties and disputes, and prevents probate court from deciding what happens. Businesses, homes, property, money, child custody, medical issues, and even funeral details can all be detailed. It is a good idea for everyone to have a will.
God has a will. He has children, and He wants them to inherit His kingdom and all its blessings. He had an inheritance for His people in the Old Testament. It was the land of Canaan, which was called the promised land because He promised it to Abraham (Gen 12-17). Property rights were very important in the Old Testament, and there were specific laws about them because their land was their inheritance from God and was to be handed down through the generations. God also called Israel His inheritance 13 times.
In the New Testament, God does not promise the church a specific tract of land for their home. He wants the church to be all over the world (Mt 28:19, Mk 16:15, Lk 24:47, Acts 1:8, Rev 7:9). He has promised the church an inheritance in His eternal spiritual kingdom (Mt 5:3 & 10, Mt 25:34, Acts 20:32, Acts 26:18, Rom 8:17, 1Cor 6:9-10, 1Cor 15:50, Gal 3:29, Gal 4:7, Gal 5:21, Eph 1:11 & 18, Eph 5:5, Col 1:12, Col 3:24, Titus 3:7, Heb 1:14, Heb 9:15, Jam 2:5). We can inherit the earth (Mt 5:5), everlasting life (Mt 19:29), promises (Heb 6:12 & 17), righteousness (Heb 11:7), grace (1Pet 3:7), a blessing (1Pet 3:9), and all things (Rev 21:7). The vineyard in the parable of Jesus is the kingdom of God. In His humanity as the Son, He is the heir (Mt 21:38). Yet He wants to share it with us. (Rev 2:26-27, Rev 3:21).
How do we become partakers of this inheritance? We must be born again (Jn 3:3-5). This involves repentance, water baptism by immersion in the name of Jesus Christ for the remission of sins, and receiving the gift of the Holy Ghost (Acts 2:38). This how we apply the death, burial, and resurrection of Jesus to ourselves (Rom 6:1-4, Col 2:8-12). The gift of the Holy Ghost is the earnest of our inheritance (Eph 1:14). When someone is purchasing a property, they are required to give earnest money to demonstrate they are committed to this purchase. Jesus purchased us with His own blood (Mt 26:28, Acts 20:28). He gives us the same Spirit that raised Him from the dead (Rom 8:9-11). It is a foretaste of what heaven will be like.
When someone makes out a last will, it is nothing but a piece of paper until they die. That is when it goes into force. This is how the New Testament also works. The time that Jesus was here on earth was a transition period between the Old and New Testaments (Mt 11:12-13, Lk 16:16). The New Testament could not go into force until Jesus shed His blood and died (Mt 26:28, Heb 9:15-17). After someone dies , the will must be proven to be valid. Jesus rose from the dead and appeared to his disciples 16 different times which proved His will (Mt 28, Mk 16, Lk 24, Jn 20-21, Acts 1, 1Cor 15). Before His ascension He instructed His disciples to go and "read" His will to anyone who would want to partake of the inheritance. Yet they were not supposed to start doing this until they received the Holy Ghost (Lk 24:46-29, Acts 1:1-8). In Acts 2, they received the Holy Ghost and spoke with tongues. Then Peter stood up and "read the will". He preached the death, burial, and resurrection of Jesus Christ, and they responded by repenting, getting baptized, and receiving the Holy Ghost. This was the beginning of the New Testament. Matthew 1:1 is the beginning of the part of the Bible we call the New Testament, but the actual covenant did not go into effect until Acts 2. The apostles continued to preach the same message everywhere, and people entered the kingdom of God the same way way (Acts 8:12-17, Acts 10:43-48, Acts 19:1-6, Acts 22:16).
People can have their will modified over time as situations change. God had a covenant with Israel called the law of Moses. This is documented in the part of the Bible we call the Old Testament. Testament, covenant, and will mean the same thing in this context. God changed the covenant by coming in the flesh, shedding His blood, and dying for our sins (Heb 8-10). This covenant is still in force today. Someday He will come again, and there will be another change. He will call His people by a new name (Is 62:2, Rev 2:17, Rev 3:12). He will make a new heaven and a new earth (Is 65:17, Is 66:22, Rev 21:1). He will restore everything lost in the garden of Eden (Is 11, Acts 3:21, Rom 8:19-23, Rev 21:1-5). However, through His salvation we can partake of the newness of the kingdom of God now as well (2Cor 5:17).