This is a warning Jesus had to give to the leader of a church, "I know your deeds, that you are neither cold nor hot; I wish that you were cold or hot. So because you are lukewarm, and neither hot nor cold, I will spit you out of My mouth" (Re.3:15,16). The human way of looking at things is that to be lukewarm is better than being cold. But Jesus thinks that if anyone claims to follow Him and yet takes things lightly, he hasn't really come to know Him. We can understand it when one is cold towards Jesus. But we need to realise that if we don't see Jesus as the pearl of greatest value and therefore become willing to give up everything else for Him (Mt.13:45,46), we haven't really seen Him at all.
How can we see if we have become lukewarm towards Jesus? That is when we take Him for granted and our relationship with Him for granted. Maybe we look back to a certain date when we believed Him. But now we just assume we are following Him and that He is there for us. We go to Him only when we are in need. We may be reading the Bible and going to the church out of a sense of duty rather than with any longing or eagerness on our part. When we have read the Bible we feel good about it, and we go to church because we don't want anyone to think we have backslidden. But otherwise, we hardly think about Jesus or share our life with Him, and generally carry on just like anyone who does not know God.
On the contrary, what would a man do who is 'hot' for Jesus? His life centres around Jesus! He is always talking to Him, asking Him for wisdom and strength, confessing his failures, thanking Him when things go well, always wanting to know Him better, to be pleasing to Him and to fulfil His plans for his life, and to tell others about Him. He is eternally grateful to the Lord that He has had mercy on him, and he wants to live a life that always expresses his gratitude.
Such a person who is hotly in love with Jesus can indeed backslide when things go well for him and he doesn't feel any urgent need to seek His help. This is in our human nature. God warned the people of Israel this could happen to them when He blessed them (De.8:11-14). The danger is that when it happens we may not recognise it. We may be looking back to the date of our coming to the Lord, all the blessings He has given, how we do the usual 'religious' things connected to our faith, etc. We may not even recognise it if we become 'dead' in our heart towards the Lord as we pat ourselves over the 'ministry' we are doing for the Lord (Re.3:1).
What the Lord wants is for us to be able to see our true condition (v.18). Only then we can know if we have backslidden from a first love for Jesus (Re.2:4), or if we have never known Him at all in a real sense but we have been assuming we were Christians. If we want to be truly pleasing to the Lord, let us make sure that we are on fire for Him. Every part of our life should show the effect of this relationship with Him.