Taken out of context and without scriptures that show to the contrary, this is a common explanation of this scripture.
However, if you think about this, it doesn't make much sense. Jesus is saying this to the Pharisee's. But he also said:
Mat 23:27 Woe unto you, scribes and Pharisees, hypocrites! for ye are like unto whited sepulchers, which indeed appear beautiful outward, but are within full of dead men's bones, and of all uncleanness.
Mat 23:28 Even so ye also outwardly appear righteous unto men, but within ye are full of hypocrisy and iniquity.
Hypocrisy and lawlessness were within the Pharisee's, not the kingdom of God.
An explanation that makes more sense is that the word translated "within" can just as easily be translated "among" or "in the midst of". Several translations render it this way:
(Bible in Basic English) And men will not say, See, it is here! or, There! for the kingdom of God is among you
(Contemporary English Version) There is no use saying, 'Look! Here it is' or 'Look! There it is.' God's kingdom is here with you."
(Darby) nor shall they say, Lo here, or, Lo there; for behold, the kingdom of God is in the midst of you.
(EMTV) neither will they say, 'Look here!' or 'Look, there it is!' For behold, the kingdom of God is among you."
(ESV) nor will they say, 'Look, here it is!' or 'There!' for behold, the kingdom of God is in the midst of you."
(ISV) People won't say, 'Look! Here it is!' or "There it is!' For the kingdom of God is among you."
(MKJV) Nor shall they say, Lo here! or, behold, there! For behold, the kingdom of God is in your midst.
What Jesus was likely telling them was that he, as the Christ, represented the kingdom of God. You can't have access to the Kingdom of God unless you go through Christ. So Christ was standing right in their midst and they couldn't even observe it.
The Lord loves all and wishes salvation for all... Jesus answered and said to him, "If anyone loves Me, he will keep My word; and My Father will love him, and We will come to him and make Our home with him. (John 14:23)
I agree with this. But it isn't a free ticket to salvation. Other scriptures make clear that obedience is a clear requirement:
Heb 5:9 And being made perfect, he became the author of eternal salvation unto all them that obey him;
Rom 6:16 Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness?
In John 14:23, 'keep' means 'obey'.