Why is being 'in Adam' the beginning of all our trouble?

Les explains that the entire sin problem began in Adam, and that every person was in Adam so far as God is concerned: 'We were in Adam.' That identification is the beginning of all our trouble. He then contrasts it with Christ's finished work: when Christ died, He completely fulfilled everything a holy God could demand, so everything we need has already been accomplished. The veil was rent from top to bottom, opening the way into the holiest and unveiling God's righteousness. Justification is God's judicial act whereby He declares the sinner just as if he had never sinned. Sin is still sin, and God has not changed, so the problem that began in Adam is real—but the remedy is not human effort; it is what Christ has already done.