3:15 ‘I know your deeds, that you are neither cold nor hot. I wish you were either cold or hot! 3:16 So because you are lukewarm, and neither hot nor cold, I am going to vomit you out of my mouth!’

As popularly understood:

Cold = decidedly opposed to the things of God
Hot = enthusiastically committed to the things of God
Lukewarm = indifferent to the things of God

The implication would therefore be: better to be out and out against God than to sit on the fence.

But it is probably a misreading of this message to understand it as teaching that it is better to be ‘cold-hearted’ towards God than ‘luke-warm’ towards him; that out-and-out atheism, say, is better than complacent agnosticism. There may be some truth in such assertions, but it is not the truth of this passage.  Both hot and cold water are good: hot water for bathing, and cold water for drinking.  But lukewarm water is good for nothing.

Leon Morris allows for (without committing himself to) the ‘popular’ interpretation, outlined above:

‘The words may mean that the Christianity of this church was lukewarm, when the meaning would be ‘outright denial is better than phoney piety’ (Orr). To prefer a rejection of the faith to the way the Laodiceans professed it is startling to say the least (cf. 2 Pet. 2:21). But to profess Christianity while remaining untouched by its fire is a disaster. There is more hope for the openly antagonistic than for the coolly indifferent. ‘There is no one farther from the truth in Christ than the one who makes an idle profession without real faith’ (Walvoord). Their coolness was a denial of all that Christ stands for.’

Some background it helpful here:

‘Cold water (and sometimes spiced hot water) was preferred for drinking, and hot water for bathing, but Laodicea lacked a natural water supply. Water piped in from hot springs six miles to the south, like any cold water that could have been procured from the mountains, would be lukewarm by the time it reached Laodicea. Although water could be heated, the natural lukewarmness of local water (in contrast with the hot water available at nearby Hierapolis) was undoubtedly a standard complaint of local residents, most of whom had an otherwise comfortable lifestyle. (Their imported water was also full of sediment, though better, said the geographer Strabo, than the water of Hierapolis.) Jesus says: “Were you hot [i.e., for bathing] or cold [i.e., for drinking], you would be useful; but as it is, I feel toward you the way you feel toward your water supply-you make me sick.”’ (NT Background Commentary)

According to Harper’s Bible Commentary:

‘This metaphor for ineffectiveness has been linked to the region’s water supply. The “hot” springs of Hierapolis were famous for their medicinal properties, and the “cold” waters of Colossae were prized for their purity. The tepid waters of Laodicea, however, were both abundant and bad.’

Ian Paul has a thorough discussion of this passage.  He notes that the risen Jesus appears to value cold and hot water equally.  It is therefore unconvincing to argue that ‘coldness’ (understood as decided antagonism towards Christ) is is relatively good.  Movoever, writes Ian Paul, this understanding views ‘hot’, ‘cold’ and ‘lukewarm’ as inner dispositions towards God and faith, whereas the emphasis, throughout this passage, is on deeds; on a changed life.  Ian Paul almost makes the comment – made by others, too – that Laodicea lacked the hot water that supplied Hierapolis (6 miles away), and also the cold water enjoyed by Colossae (ten miles distant).  Hot water was useful for healing and therapy; cold water for quenching thirst.  The water reaching Laodicea was, indeed, lukewarm and, being laden with minerals, was good for nothing, apart from being an emetic.

See this article by Stanley E. Porter, and also chapter 39 of Urban Legends of the New Testament, by David A. Croteau)