From the book On the Holy Spirit by Saint Basil the Great, bishop
The Spirit Gives Life
Our Lord made a covenant with us through baptism in order to give us eternal life. There is in baptism an image both of death and of life, the water being the symbol of death, the Spirit giving the pledge of life. The association of water and the Spirit is explained by the twofold purpose for which baptism was instituted, namely, to destroy the sin in us so that it could never again give birth to death, and to enable us to live by the Spirit and so win the reward of holiness. The water into which the body enters as into a tomb symbolizes death; the Spirit instills into us his life-giving power, awakening our souls from the death of sin to the life that they had in the beginning. This then is what it means to be born again of water and the Spirit: we die in the water, and we come to life again through the Spirit.
To signify this death and to enlighten the baptized by transmitting to them knowledge of God, the great sacrament of baptism is administered by means of a triple immersion and the invocation of each of the three divine Persons. Whatever grace there is in the water comes not from its own nature but from the presence of the Spirit, since baptism is not a cleansing of the body, but a pledge made to God from a clear conscience.
As a preparation for our life after the resurrection, our Lord tells us in the gospel how we should live here and now. He teaches us to be peaceable, long-suffering, undefiled by desire for pleasure, and detached from worldly wealth. In this way we can achieve, by our own free choice, the kind of life that will be natural in the world to come.
Through the Holy Spirit we are restored to paradise, we ascend to the kingdom of heaven, and we are reinstated as adopted sons. Thanks to the Spirit we obtain the right to call God our Father, we become sharers in the grace of Christ, we are called children of light, blessing is showered upon us, both in this world and in the world to come. As we contemplate them even now, like a reflection in a mirror, it is as though we already possessed the good things our faith tells us that we shall one day enjoy. If this is the pledge, what will the perfection be? If these are the firstfruits, what will the full harvest be?