You are not reformed in doctrine enough, as like her you perpetuate a tradition of men for which you try to extrapolate support from Scripture for. Which attempts are more incongruous for a reformed, as you hold to SS, yet while the Holy Spirit very clearly presents repentance and faith as preceding baptism, and provides many examples of baptism, yet He never describes infants being baptized (and thus must presume whole household baptisms must include children under about 4).
Doctrine may be established upon principals flowing from what is implicit in precepts and precedents, but the conspicuously absence of any infant baptism when so much is said about baptism is consistent for doctrine of a basic practice.
In addition the correspondence of circumcision to baptism is very limited, as while the former answers to the latter as regards being a sign of being in covenant, the former - besides only being for males - was distinctly required for infants, even to the very day it was to be done, and thus did not require faith (though one could be a believer), while repentance and faith is distinctly required for baptism.
Believer baptism is not the plain teaching of scripture.
What?! Now you really sound Catholic, denying what is plainly exampled in Scripture in order to keep your tradition which is plainly unseen in the NT.
Then Peter said unto them, Repent [which is a decision of faith], and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. (Acts 2:38)
Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls. (Acts 2:41)
But when they believed Philip preaching the things concerning the kingdom of God, and the name of Jesus Christ, they were baptized, both men and women. (Acts 8:12)
And as they went on their way, they came unto a certain water: and the eunuch said, See, here is water; what doth hinder me to be baptized? And Philip said, If thou believest with all thine heart, thou mayest. And he answered and said, I believe that Jesus Christ is the Son of God. (Acts 8:36-37)
Can any man forbid water, that these should not be baptized, which have received [by faith] the Holy Ghost as well as we? (Acts 10:47; cf. 15:7-9)
And Crispus, the chief ruler of the synagogue, believed on the Lord with all his house ; and many of the Corinthians hearing believed , and were baptized. (Acts 18:8)
Then said Paul, John verily baptized with the baptism of repentance, saying unto the people, that they should believe on him which should come after him, that is, on Christ Jesus. When they heard this, they were baptized in the name of the Lord Jesus. (Acts 19:4,5)
It’s hardly a surprise that we see adults who were already part of the old covenant via circumcision getting baptized into the new covenant as believers.
And it is hardly a surprise that we see adults who were Gentiles and outside the old covenant getting baptized after they believed and were born again and were part of the body of Christ, God "purifying their heart by faith." (Acts 10:34-47; 15:9)
Abraham was a believer when he was circumcised.
But children were not, yet were commanded, not to repent, believe and be circumcised as is the case under the New Cov, but to be circumcised the 8th day.
it would be absurd to include children in the old covenant but reject them in the new.
Indeed, yet while circumcision placed souls into a physical theocractic community whether they believed or not, and which consisted of lost and saved souls, in contrast, baptism requires and signifies faith which places one in a spiritual body which only consists of true believers. The act of baptism apart from personal faith gets the individual wet, not washed from sin, which is by faith.
To him give all the prophets witness, that through his name whosoever believeth in him shall receive remission of sins. While Peter yet spake these words, the Holy Ghost fell on all them which heard the word. (Acts 10:43-44)
And when there had been much disputing, Peter rose up, and said unto them, Men and brethren, ye know how that a good while ago God made choice among us, that the Gentiles by my mouth should hear the word of the gospel, and believe. And God, which knoweth the hearts, bare them witness, giving them the Holy Ghost, even as he did unto us; And put no difference between us and them, purifying their hearts by faith. (Acts 15:7-9)
Meanwhile, infants do not need to believe on Christ as they are not accountable for sin, and what one did himself is the basis for damnation, (Rv. 20:11-14) not what one's father did. (Dt. 24:16; 2Ki 14:5,6; 2Ch 25:4; Jer 31:29,30; Eze 18:20)
Do you even know what reformed means?