So, what you were saying (in a now-deleted post) is that the new covenant and how to enter it—responding rightly to the gospel message—did not happen until after the cross.
This is biblically correct in one sense, namely, the new covenant began with the cross of Christ. However, responding rightly to the message of the gospel (i.e., faith) is not what puts a person in the new covenant. Faith is the instrument by which we receive Christ; it is not the cause of covenant inclusion.
Let’s walk through this carefully. The cross of Christ established the new covenant (Luke 22:20; Heb 9:15-17). This new covenant, as the historical administration of the one covenant of grace, is objectively ratified by his blood and now exists as a reality.
But how does one enter that covenant relation? Through union with Christ. Every spiritual blessing of the new covenant—election, redemption, forgiveness, inheritance, etc.—is located “in Christ” (Eph 1:3–14). There are no covenant benefits distributed independently of union with Christ. To be in Christ is to be in the covenant; those not in Christ are outside the covenant.
So, how does union with Christ happen? Scripture is explicit that this union is wrought by the Holy Spirit. As Paul tells us, it is by one Spirit that we are all baptized into one body (1 Cor 12:13), and anyone who doesn’t have the Spirit does not belong to Christ (Rom 8:9). It is by the Spirit that we are incorporated into Christ’s body. Belonging to Christ is covenant membership, which is effected by the Holy Spirit. It is not a matter of human assent or moral resolve.
Entrance into the new-covenant kingdom presupposes regeneration. “Unless a person is born of water and Spirit, he cannot enter the kingdom of God.” According to Old Testament passages which Nicodemus should have known (e.g., Isa 44:3-5; Ezek 36:25-27; 37:9-10), both water and wind function as figures that represent the regenerating work of the Spirit. The same causal order appears in Johannine theology more broadly. “Everyone who believes that Jesus is the Christ has been born of God” (1 John 5:1). The Greek grammar makes it clear: believing is the evidential expression of divine begetting, not its cause. “The one who belongs to God listens and responds to God’s words. You don’t listen and respond, because you don’t belong to God” (John 8:47).
This, then, is the biblical sequence: Christ establishes the covenant and all its benefits in himself (Eph 1:3; Heb 9), the Spirit regenerates the elect sinner, creating new life in union with Christ (John 1:13; John 3:3-8; Eph 2:5; Titus 3:5; 1 Cor 12:13; Rom 8:9-11), and that new life expresses itself in faith, which receives and rests in Christ (John 6:37; Phil 1:29; Heb 4:10) and all the salvific benefits in Christ (Rom 6:3-5; 1 Cor 1:30; Eph 1:3-7).
Faith does not put one in the new covenant; it lays hold of the One in whom that covenant relationship already exists, a union effected by the Holy Spirit in everyone the Father gives to the Son. Apart from union with Christ, faith would never arise, for faith is the expression of divine begetting.
If responding rightly to the gospel is made the condition that puts one in the covenant, several problems follow, the thorniest of which is that it implies old-covenant saints were not united to Christ—an error that you explicitly affirm (“[In Acts 2] the doors to the church opened for the first time”). Scripture does not allow this. Abraham believed the gospel beforehand (Gal 3:8), was justified by faith, and shared in the same covenant of grace. What changed at the cross was not the way sinners are united to Christ, but the historical completion of the work to which their faith pointed. (The covenant signs changed, too. Old covenant signs were typological and promissory, prefiguring Christ who was to come, whereas new covenant signs are sacramental and participatory, proclaiming Christ until he returns.)
If I may quote something you said, “I am really surprised that I have to explain this. I thought this was elementary.”
Verse 13
For in one Spirit were we all baptized into one body, whether Jews or Greeks, whether bond or free; and were all made to drink of one Spirit.
In one Spirit were we all baptized ... Throughout the New Testament, Christian baptism is revealed to be one of the two essential elements of the new birth, without which no man may see the kingdom of God. These are: obedience to the ordinance of baptism and the reception of the Holy Spirit. Jesus joined these two essential elements by his requirement that people be "born of the water and of the Spirit" (
John 3:5ff). Peter joined them on Pentecost by the command that all people should "repent and be baptized ... and ... receive the Holy Spirit" (
Acts 2:38ff). There is no doubt whatever that Paul's words here refer to the same twin essentials of the new birth, the same being a prior condition of participation in the body of Christ.
In one Spirit ... As Kelcy said, `This is actually `by one Spirit,' making the Holy Spirit the agent or administrator of baptism."
19 In a similar way, Christ was named as the actual administrator of the rite of baptism, even though his disciples actually did the baptizing (
John 4:1,
2). The unity of the godhead makes it correct to refer any action ordained and commanded by God, to the Father, the Son, or the Holy Spirit; and when the action is obeyed, it is proper to say that any one of them did it. This truth does not exclude the reception of the indwelling Spirit in Christian hearts, as Paul dogmatically emphasized that in the very next clause, "made to drink of one Spirit."
We were all baptized ... and were all made to drink of one Spirit ... As Metz correctly noted, "the word `baptized' relates to the actual act of baptism."
20 The mention of the Spirit as the administrator of baptism in this verse provoked Hodge to declare that the baptism in view, therefore, is "the baptism of the Holy Ghost!"
21 If that is true, it would make Paul here declare that all of the Corinthians were baptized in the Holy Ghost, or had received the Holy Spirit baptism! Who could believe such a thing? It is true of course that all of them had themselves baptized, and in consequence had all received the gift ordinary of the Holy Spirit, common to all Christians; but to suppose that those carnal Corinthians had "all" participated in the baptism of the Holy Spirit is impossible. Of course, the design of many scholars is to get water baptism out of this text altogether; but that is also impossible.
All made to drink of one Spirit ... This refers to the reception of the ordinary gift of the indwelling Spirit by the Corinthians in consequence of primary obedience to the gospel. "There is no evidence that all the disciples at Corinth, or any of them, had been baptized in the Holy Spirit."
22
From Coffman's commentary on the bible located online in Studylight.org