I've come to believe that one of the  most significant theological controversies the modern church must acquaint itself with is the controversy surrounding the Marrow of Modern Divinity. The issues concerning Law and Gospel, legal and evangelical repentance, and the free offer of the Gospel to the unconverted are all bound up with the Marrow Controversy. Thomas Boston, and Ralph and Ebenezer Erskine are among the more well known Marrowmen. You can find Christian Focus' newly reprinted edition of the Marrow of Modern Divinity (with Boston's Notes and introductions by Phil Ryken and William Vandoodewaard) here. I would also be remiss if I did not recommend Sinclair Ferguson's outstanding lectures on the Marrow. You can find them here. You can also find other helpful lectures here and here. Perhaps the most careful treatment of the Marrow controversy in print is William Vandoodewarrd's The Marrow Controversy and Seceeder Tradition. Please spend the time familiarizing yourself with the Marrow theology.