Our spiritual union with Christ is both invisible and mysterious, but that does not mean that it is unreal. Spiritual things, in their own sphere, are just as real as material things. Moreover, our union with Christ is also unbreakable and permanent. The person who is truly joined to Christ will always be joined to Christ; therefore the catechism adds the word inseparably.

J. G. Vos, The Westminster Larger Catechism: A Commentary, 145.