I will gather all the nations and bring them down to the Valley of Jehoshaphat. And I will enter into judgment with them there, on behalf of my people and my heritage Israel, because they have scattered them among the nations and have divided up my land (Joel 3:2 ESV)
Put in the sickle, for the harvest is ripe. Go in, tread, for the winepress is full. The vats overflow, for their evil is great. Multitudes, multitudes, in the valley of decision! For the day of the Lord is near in the valley of decision. (Joel 3:13-14)
Just one chapter on from that great prophetic declaration of what the Lord would do at Pentecost, we see another example of God drawing and separating the peoples of the nations. It is a passage of judgement, but also one of harvest and salvation. God draws all men to himself, but separates them in judgement based on the decision they make in relation to his Son — Jesus. So it will be in the final judgement (Mt 13:47-50)
At Pentecost too, we see a great harvest as God draws the peoples of the nations and then separates them. If the way the church was born is a pattern for how it should grow then we should expect this process of drawing and separating to continue. There is no substitute for the message of the gospel. It was Peter's declaration that cut the people to the heart and caused them to call on the Lord for their salvation. But was the baptism in the Spirit and the tongues that accompanied it just a side show, an optional extra, or an integral part of the process? And does God equip his church with gifts today as a fringe benefit or as tools to get the job done?
Theologians have debated (and still do!) as to whether the tongues that were given, and still are given, were for the edification of believers, or the evangelisation of unbelievers. I have no doubt as to the former, and if tongues truly is a sign for unbelievers, and its purpose towards them is to draw and separate, the process in which the church was born and continues to grow, then the answer might not be an either-or.