Humans are not angelic beings. We belong to a special kind of creation made out of matter and spirit. A human cannot be considered such in a disembodied state or as unanimated matter, and that is why we await the resurrection of the dead at the Final Judgement, where reward and penance will be given to humans in the same condition, they lived in.
As a unity of matter and spirit, humans can be affected in different ways through physical and spiritual stimulus. Whatever stimulates the spirit will have material consequences, and vice versa. That is why the people of God have always practiced fasting and ascetism, to submit the flesh and the spirit to the Will of God (Matthew 3:4; Luke 5:35).
Knowing this, as our Creator, Lord Jesus established since the beginning of mankind, different physical acts that reflected spiritual realities and our covenants with God, to the benefit of our beings as a whole.
As soon as Adam and Eve fell, God established animal sacrifice as atonement for sins (Genesis 4:4); He forbade humanity to consume animal flesh with its blood in recognition of God’s sovereignty over all living beings (Genesis 9:4); and He ordered circumcision as a sign of the Abrahamic covenant (Genesis 17:10).
Certainly, all these things could have been given in faith and theory. Really God does not need animal blood to forgive (Hebrews 10:4); or a faithful man to stop from consuming his hunt with its blood to acknowledge God’s ownership of the life he has taken as food; and neither needs the cut off skin to honour a covenant based primarily on Faith (Romans 4:10-12); but God used these physical means to make us understand the depth of the spiritual realities they represented, and through our faithful obedience, to channel grace over us.
In the same way, Lord Jesus established Baptism as the doorway to a new life as a Child of God; and the Last Supper as a memorial of his Atoning Sacrifice, to be partaken with faith and devotion, for the blessings of our whole beings, spiritual and physical.
Certainly salvation can be achieved without the water of baptism, the same way as a person could be counted as ‘Just” without circumcision (Romans 2:27;1Corinthians 7:19; Philippians 3:3; Colossians 2:11); but that is the way God wanted us to seal our commitment to Him.
In the same way, Lord Jesus established the Eucharist, as an ongoing sacrament based on the giving of his life on the Cross, his flesh and blood, to physically experienced the spiritual reality of being part of Him in the flesh as much as in the spirit. This sacrament that embodies the apex of the salvific act is repeatable for this reason, to sanctify us every time we partake from it, channelling sanctifying grace and forgiveness of sins, and to help us to continue our spiritual communion with Christ until we die or He comes back again in Glory.
Sacraments are much more than just a symbol or a memorial. They were given to us for a reason, and they are unnegotiable; they must be celebrated as commanded by our Lord. They are the physical connections our invisible God, left for us to remember his incarnation and historical reality, and the salvation of ourselves.