Explain why god said those in Christ will rise first and those alive will follow after. If people in Christ are left behind?
Depends on which epistle and chapter you are referring to.
Example... if you are referring to this reference;
1 Thessalonians 4:14 For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him. 15 For this we say unto you by the word of the Lord, that we which are alive and remain unto the coming of the Lord shall not prevent them which are asleep. 16 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: 17 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord. 18 Wherefore comfort one another with these words.
Is that contingent on what Paul had shared at the beginning of the chapter? If the readers are agreeing with Paul for how we ought to walk in Him, then they understand the promise given at the end of the chapter for abiding in Him and His words.
1 Thessalonians 4:1Furthermore then we beseech you, brethren, and exhort you by the Lord Jesus, that as ye have received of us how ye ought to walk and to please God, so ye would abound more and more. 2 For ye know what commandments we gave you by the Lord Jesus. 3 For this is the will of God, even your sanctification, that ye should abstain from fornication: 4 That every one of you should know how to possess his vessel in sanctification and honour; 5 Not in the lust of concupiscence, even as the Gentiles which know not God: 6 That no man go beyond and defraud his brother in any matter: because that the Lord is the avenger of all such, as we also have forewarned you and testified. 7 For God hath not called us unto uncleanness, but unto holiness.