Believe me when I say that I am in the Father and the Father is in me; or at least believe on the evidence of the miracles themselves. (NIV)
This verse is sometimes used to prove the Trinity, but it proves nothing of the kind. The exact same language about being “in” is used many times of Christians. We assert that when the same exact language is used both of Christ and of Christians, it needs to be understood the same way. We are “in” Christ, and Christ is “in” us (cp. John 14:4-7; 17:21,23 and 26). When used in the sense of “in God,” or “in Christ,” the word “in” refers to a close communion, a tight fellowship. It was part of the covenant language of the day, when people spoke of being either “in” or “cut off from” the covenant.
Morgridge, pp. 116 and 117
Racovian Catechism, pp. 142 and 143