Since not everyone who is baptized into the visible Church is predestined to heaven, and some who are not baptized are indeed predestined to heaven, many people are skeptical of the Christian claims about baptism and the importance of membership in the visible church.
But in many ways, their claims against baptism and church membership are also claims against professing faith in Christ.
I say “many ways” because there is a difference. For those who reach the age of maturity, I don’t believe anyone is predestined to heaven who is not also predestined to profess faith in Christ as a means of reaching that fore-ordained destiny.
But on the flip side, most of us know people who have professed faith and then later proven false, just as we know people who have been baptized and who have proven false, and who have been members of the the visible church, which is the house and family of God out of which there is ordinarily no salvation, and who have proven false.
If we are going to preach against baptismal efficacy, not only should we stop claiming to be Presbyterians, but we should also preach against professing faith in Christ as a basis for assurance.
Being skeptical of baptism means being skeptical of faith in the only form that it is publicly known.
And just so we are clear: who does and does not counts as a believer, is supposed to be a public fact, not a hidden secret. The Apostle Paul could not write “let us not pass judgment on one another any longer, but rather decide never to put a stumbling block or hindrance in the way of a brother,” (Romans 14.13) it who was a believer was some kind of mystery. Likewise, Paul said he knew who was a member of the visible church by who was baptized.
“Now you are the body of Christ and individually members of it.” — 1 Corinthians 12.27
“For just as the body is one and has many members, and all the members of the body, though many, are one body, so it is with Christ. For in one Spirit we were all baptized into one body—Jews or Greeks, slaves or free—and all were made to drink of one Spirit. For the body does not consist of one member but of many.” — 1 Corinthians 12.12-14
Here is a brief paper I wrote to present the teaching of the Westminster Confession, and Larger and Shorter Catechism, on the sacramental efficacy (which, in case this is not clear to you, includes baptismal efficacy). Naturally, this is not really as important as arguments from the Bible. But my concern at the time was a lack of knowledge among those who claimed to subscribe to Westminster.
Also here are some great things you can read about baptism:
Analogies to baptism by Peter Leithart
Baptism and the Church by Peter Leithart
Finally, here is a great little piece about assurance in the context of all God’s public means of grace, and also the sufficiency of faith for salvation:
Overheard in a Pastor’s Study by Peter Leithart