Modern paedobaptsits have been arguing with eachother for decades about union with Christ. They each have a piece of the puzzle, but they can’t put the whole puzzle together because of their unbiblical covenant theology. The solution to the “union with Christ” conundrum is that union with Christ is the covenant of grace (new covenant). Here are some words from Keach (point 4), where he appeals to and quotes Nehemiah Coxe to explain New Covenant union.
Take heed on what you build your Hopes of Justification and Salvation, what is that which bears up your Spirits: for if you are Trees that grow not out of the true root, Jesus Christ, and the Covenant of Grace; if you have not Union with the Lord Jesus, or are not built on that Foundation, or Corner-stone God hath laid in Sion, down you fall; for now the ax is laid to the Root of the Trees…
Now, from the whole, it seems to me to be a strange Thing, which is lately asserted, viz.
‘That the Infant Seed of Believers, (during their Infancy) have all of them a certain Interest in the Covenant of Grace: By vertue of which, they are compleatly Justified before God, from the Guilt of Original Sin, both Originans, and Originations; and yet, when they come to Years of Discretion, may (yea must) by their actual closing with, or refusing the Terms of the Covenant, either obtain the continuation, and confirmation of their Covenant Interest, or be utterly, and finally cut off from it, and so perish Eternally in their Ignorance of God, and Rebellion against him.
Answer, To which I must say, That they seem to make the Covenant of Grace, such a Conditional Covenant, that renders it in Nature and Quality, like the Sinai Covenant, or Covenant of Works, i.e. If they perform the Righteousness required, they shall live; if they Obey not, or make not Good, this pretended Covenant of Grace, they shall dye, or be cut off: Let our Brethren, who are found in the Doctrine of Free-Grace, consider this.
2. And as the Promises of the New Covenant, will admit of no such partial Interest, (saith a Learned Author) so neither can this Opinion consist with the Analogy of Faith, in other Respects; for either the stain of Original Sin, in these Infants is purged, and the dominion of Concupiscence in them destroyed, when their Guilt is pardoned, or it is not; if it be, then the Case of these Infants, in point of Perseverance, is the same with Adult Persons, that are under Grace, by actual Faith; and then a final Apostacy, from the Grace of the New Covenant, must be allowed to befall the one, as well as the other, notwithstanding all Provisions of that Covenant, and Engagement of God therein, to make the Promise sure to all the Seed, Rom. 4:16.
But this the Author will not admit: If he say, That their Guilt is pardoned, but their Natures are not changed, or renewed, nor the Power of Original Corruption destroyed, so as that Sin, shall not have Dominion over them; it will be replyed, That then, notwithstanding their supposed Pardon, they remain as an unclean Thing, and so uncapable of admission into the Kingdom of God. Thus this worthy Author.
3. To which let me add, Certainly if Divine Habits were in those Infants, they would immediately be manifested; or be sure when they are grown up, would appear in them by gracious Operations flowing from thence: But since those Acts, or Products of such a gracious Habit, appear not in them, ’tis evident, they never had them infused.
4. All that are in the Covenant of Grace, (if they live) the Fruits of Faith and Holiness, will flow naturally from those sacred Habits, God hath by his spirit planted in them, as heat and light doth from the Fire, when ’tis kindled on the Hearth. The Truth is, such who are united to Christ, and have Faith in him, and so are actually in the Covenant of Grace, are also washed and purged from Sin, and Pollution, see Ezek. 16; Rom. 5:14; Act. 15:10. None can have Union with Christ, but by the in-dwelling of the Holy Spirit; and wheresoever the Spirit of Christ is, it applies, The Blood of the Covenant, not only for Pardon, but also for the purging the Conscience from dead Works, to serve the living God: ‘And therefore, (as the same Learned Author observes) as certain as any derive a New Covenant Right from Christ for Pardon, they also receive a vital Influence from him, for the renovation of their Natures, and conforming their Souls to his Image: Therefore, to assert, That the Grace of Christ, is applied to some, for remission of Sins only, or that the guilt of any Sin, can be pardoned to any Person, and yet that Sin retains its Dominion over them, is a Doctrine, I understand, not to be sound, or agreeable to the Doctrine that is according to Godliness.
Keach, B. (1693). Sermon III. In The Ax Laid to the Root, Parts I & II (Vol. 2, pp. 12–13). London: John Harris.
Compare the above with Coxe on New Covenant Union with Christ.