The Protestant denominations that reject infant baptism reject it because they believe that baptism is the mark of justification, whereas biblically we know that election is the mark of justification.
Baptism is only an acknowledgement that the candidate is entitled to drink from the Rock :
1 Corinthians 10:1-6 NET For I do not want you to be unaware, brothers and sisters, that our fathers were all under the cloud and all passed through the sea, and all were baptized into Moses in the cloud and in the sea, and all ate the same spiritual food, and all drank the same spiritual drink. For they were all drinking from the spiritual rock that followed them, and the rock was Christ. But God was not pleased with most of them, for they were cut down in the wilderness. These things happened as examples for us, so that we will not crave evil things as they did.
Children are automatically included in the table fellowship from birth, moving from milk food to solid food. At the appropriate time, Christians, both born believers and converts will be "called", but only a few are "chosen" :
Matthew 22:11-14 NET But when the king came in to see the wedding guests, he saw a man there who was not wearing wedding clothes. And he said to him, ‘Friend, how did you get in here without wedding clothes?’ But he had nothing to say. Then the king said to his attendants, ‘Tie him up hand and foot and throw him into the outer darkness, where there will be weeping and gnashing of teeth!’ For many are called, but few are chosen.”
Hebrews 2:1-4 NET Therefore we must pay closer attention to what we have heard, so that we do not drift away. For if the message spoken through angels proved to be so firm that every violation or disobedience received its just penalty, how will we escape if we neglect such a great salvation? It was first communicated through the Lord and was confirmed to us by those who heard him, while God confirmed their witness with signs and wonders and various miracles and gifts of the Holy Spirit distributed according to his will.
Israel witnessed the signs that confirmed Moses's message. Based on that they left serving Egypt. While learning to serve God, they felt it too uncertain and hard, even though God promised to place them in a land filled with milk and honey. Their unbelief led to hardening and rejection. Caleb's belief led to strengthening and election.
Hebrews 3:7-14, 16-19 NET Therefore, as the Holy Spirit says,
“Oh, that today you would listen as he speaks!
“Do not harden your hearts as in the rebellion, in the day of testing in the wilderness.
“There your fathers tested me and tried me,and they saw my works for forty years.
“Therefore, I became provoked at that generation and said, ‘Their hearts are always wanderingand they have not known my ways.’
“As I swore in my anger, ‘They will never enter my rest!’”
See to it, brothers and sisters, that none of you has an evil, unbelieving heart that forsakes the living God. But exhort one another each day, as long as it is called “Today,” that none of you may become hardened by sin’s deception. For we have become partners with Christ, if in fact we hold our initial confidence firm until the end. For which ones heard and rebelled? Was it not all who came out of Egypt under Moses’ leadership? And against whom was God provoked for forty years? Was it not those who sinned, whose dead bodies fell in the wilderness? And to whom did he swear they would never enter into his rest, except those who were disobedient? So we see that they could not enter because of unbelief.
Baptism is the invitation that a person can fellowship in the Body of Christ, extended to both children and converts. Later, the call is to discipleship, to overcome the enemy and enter God's rest. Many are called, but only a few are chosen. If you want to be chosen, believe! Believe God's rest is a land of milk and honey. Then your eyes will be opened.
Galatians 3:2 NET The only thing I want to learn from you is this: Did you receive the Spirit by doing the works of the law or by believing what you heard?
John 3:3 NET Jesus replied, “I tell you the solemn truth, unless a person is born from above, he cannot see the kingdom of God.”
John 3:6-7 NET What is born of the flesh is flesh, and what is born of the Spirit is spirit. Do not be amazed that I said to you, ‘You must all be born from above.’