What They Don't Do
Being baptised and taking communion do not make someone a Christian. Nor do they take away sin. The verse above is about Abraham. His sins were forgiven through faith in the Lord Jesus Christ, who said to the Jews " Abraham rejoiced to see my day: and he saw it and was glad" (John 8/56). There has only ever been one way to Heaven, and that is through faith in Christ. In the Old Testament the people of God trusted in the promised Saviour to come (Moses and David were saved this way : Hebrews 11//26, Romans 4/6). We must trust in the Saviour who has come and will one day come again. But Abraham received a sign, circumcision. But the verse above shows that the sign and the reality are not the same. Many Israelites had the sign of circumcision but not the cleansing which it signifies. Outward signs, even God-given ones, do not take away sin.
In the time of Moses, four hundred years later, God gave Israel the ordinance of the Passover. As we can see, Abraham's sins were forgiven long before God ever instituted the Passover.
In the New Testament the sacraments are Baptism and the Lord's Supper (Communion). Likewise, these do no take away guilt before God. They do not bestow forgiveness.
What they do mean
Passover in the Old Testament pointed forward to Christ, the Lamb of God, who would come to take away sin. The Apostle Paul could say, "For even Christ our passover is sacrificed for us" (1 Corinthians 5/7).
So also in the New Testament the Lord's Supper points back to what Christ has done for his people. "This do in remembrance of me" (1 Corinthians 11/25). And just as circumcision in the Old Testament was a sign of cleansing from sin and union with the Lord (Genesis 17/7-11, Deuteronomy 30/6, Jeremiah 4/4), so also in the New Testament, baptism has the same meaning (Colossians 2/11-13, Romans 6/3-5, 1 Peter 3/21).
Why there are two
There are two aspects to Christ's salvation of his people. There is what Christ has done for them in bearing their sins on the cross. Then there is what Christ does to them by cleansing from sin. This includes removing the guilt of sin (when, by his Holy Spirit, they are brought to faith in Christ), and the removal of the presence of sin in their hearts and lives (which begins when someone becomes a Christian and is only completed when they reach Heaven, and is the work of his Holy Spirit).
The two sacraments are visible signs of what Christ has done for his people (the Lord's Supper) and what he does to them in cleansing from sin (baptism). The Biblical truth of redemption accomplished and redemption applied are set before our eyes in these two ordinances of Christ.
What You Must Not Do
Many of the Jews trusted in outward signs, as if their participation in them would secure acceptance with God. Christ warned such, saying, "how can ye escape the damnation of hell?" (Matthew 23/33).
You, too, must not trust in outward ordinances as if being baptised or taking communion will get you to Heaven. It won't. In fact, to take communion when you don't trust in the Saviour who died to take away sin is hypocrisy and makes you more guilty before God.
Only Christ can save. Our trust must not even be in Christ's ordinances, but in Christ himself. "If any man thirst, let him come unto me, and drink" (John 7/33).