The passage doesn't say everyone was made alive through Christ. It says "the many died". What many? The same many that by God's grace are made alive through Christ.
Paul had already said that sin entered the world and death by sin through one man. Because of that man the sentence of death has come upon all men, because all have sinned.
Romans 5:12 (KJV) Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:
But then Paul speaks of the free gift by God's grace, and how through it the offense that brought death through the one man brought death not to ALL, because Paul says, "by the offense of one MANY be dead". All mankind were assigned the sentence of death through Adam, but from ALL only MANY die. Because the gift by grace by the one man, Jesus Christ has abounded only to MANY, not to ALL.
Romans 5:15 (KJV) But not as the offence, so also is the free gift. For if through the offence of one many be dead, much more the grace of God, and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many.
By the offense of Adam death reigns over ALL mankind. But "THEY" not ALL which receive through grace the gift of righteousness shall reign in life through Jesus Christ.
Romans 5:17 (KJV) For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.)
Just as judgment by the offense of one came to ALL humanity to condemnation, so too the free gift by righteousness unto justification of life came upon ALL humanity. Does that mean every human will be saved through Christ? The answer is NO, read on, what else does Paul say.
Romans 5:18 (KJV) Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life.
Through Adam MANY, not all were made sinners, so that by the obedience of one MANY shall be made righteous. The same MANY made sinners are the MANY made righteous through Christ. Not ALL, but MANY. What MANY? The MANY who receive grace through righteousness unto eternal life by Jesus Christ our Lord.
Romans 5:19-21 (KJV) For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous. Moreover the law entered, that the offence might abound. But where sin abounded, grace did much more abound: That as sin hath reigned unto death, even so might grace reign through righteousness unto eternal life by Jesus Christ our Lord.
Not ALL humanity is made alive by Christ! To be made alive in Christ one MUST believe in Him, being born again by the gift of righteousness through Christ and the power of the Holy Spirit. Those who physically die without believing Christ for eternal life will not be among the MANY who obtain eternal life by Jesus Christ our Lord.