Christ WAS in their midst but He has risen and ascended to the right hand of the Father (Jn 14:2,3, 20:17, 6:62, Acts 1:6-11). NOW where is He? As the head of the body, the church, He is in their midst (Rev 1:13, 2:1) and rejoices over them, and also dwells in them (Rom 8:9-11). This is the kingdom of God's dear Son (Col 1:13) and that kingdom is within through God's imputed righteousness, the peace that has come through the blood of the cross and with the joy of the Holy Spirit who reveals Christ in us, the hope of glory, God's only begotten Son (Rom 14:17,18, Col 1:27).
Tell me what kingdom these were referring to when Christ was about to be taken up in the cloud...
Acts 1:1-7
1 The former treatise have I made, O Theophilus, of all that Jesus began both to do and teach,
2 Until the day in which he was taken up, after that he through the Holy Ghost had given commandments unto the apostles whom he had chosen:
3 To whom also he shewed himself alive after his passion by many infallible proofs, being seen of them forty days, and speaking of the things pertaining to the kingdom of God:
4 And, being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me.
5 For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence.
6 When they therefore were come together, they asked of him, saying, Lord, wilt thou at this time restore again the kingdom to Israel?
7 And he said unto them, It is not for you to know the times or the seasons, which the Father hath put in his own power.