Oh zone, what MUST happen If one is to receive His Spirit. By faith, right? Scripture says this. Therefore, 'upon ALL flesh,' refers to ALL flesh who receives His righteousness will then receive His Spirit.
So, as usual, stephenspeaks, says it well, as the Lord leads Him, 'the righteous people' receive the Spirit of God poured upon them, which are ALL, but, IF , for example, water is poured upon a concrete with a plant in it, the water does not make it to the plant, the plant MUST be on good soil.
Remember, 'none are righteous,' just as Romans 3 says, 'No, not one (of us).' But, by His blood, believing in His Son having died on the cross, we are IMPUTED Christ's righteousness. This is done by/from/in/within 'the heart' of EVERY believer, ohzone (Romans 10 )