What do Evangelical Christians consider "the traditions" to be in 1 Thessalonians 2:15? Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle.


What are these "traditions" to Evangelicals?
At the time Paul wrote his first letter to the Thessalonian believers, the canon of Scripture was incomplete. Its completion came decades after Paul's death and the death of many of the eyewitnesses to the life and ministry of Jesus (e.g., Peter, James, and John, Jesus' "inner circle" of disciples). Until the completion of the canon, then, what the apostles taught other believers through both the spoken and written word became the tradition to which the early believers were to hew. 

Once the apostles began to write under the influence of the Holy Spirit, I imagine believers almost immediately began copying what the apostles had written. Those who could read and write would use those writings to teach other believers who perhaps could not read or write. Either way, by oral and/or written communication the early church clung to (and then propagated) what they believed to be the accurate and reliable teaching of the apostles. I imagine some of the copies of "Scripture" became pretty dog-eared very quickly as they passed from person to person and perhaps from local churches to other local churches. 

It almost goes without saying that the Holy Spirit was active in the whole process of communicating God's word, first through the apostles, and then through the apostles' converts, some of who became elders in local churches and passed the apostles' teaching on to faithful men and women who would teach others. Paul said to Timothy:


  "The things which you have heard from me in the presence of many witnesses, entrust these to faithful men who will be able to teach others also" (2 Timothy 2:2 NASB Updated). 


And so it went. Generation after generation entrusted to faithful men what they had heard (and later, read) in the presence of first-hand witnesses initially, and then second-hand witnesses later on. Without the aid of the Holy Spirit and the gifts he bestowed on the early Christians, however, that faithful passing on of the traditions based on the apostles' teaching would never have happened. 


  "And . . . [Jesus] gave [to the church] some as apostles, and some as prophets, and some as evangelists, and some as pastors and teachers, for the equipping of the saints for the work of service, to the building up of the body of Christ; until we all attain to the unity of the faith, and of the knowledge of the Son of God, to a mature man, to the measure of the stature which belongs to the fullness of Christ" (Ephesians 4:11-13). 


Just a few short decades after Jesus' resurrection and ascension, Christians had spread the glad tidings (like good Evangelicals!) far and wide, throughout Judea and Samaria and to the uttermost parts of the then-world, in obedience to their Lord's command to go and make disciples of all people groups. 

Today, there are as many traditions among Christians as there are denominations, including those Christians who actually believe they are non-denominational! Despite the multiplicity of traditions, however, the vast majority of true Christians, despite their denomination loyalties, find common ground and agreement in a handful of non-negotiable doctrines and teachings found in the Judeo-Christian Scriptures. A short list of them would include at least the following:


the deity of Jesus Christ, who alone is and ever shall be the image of the invisible God, sharing all the attributes of both the Father and the Spirit
the virgin birth of Jesus, the "seed of the woman," and his sinless life
the sacrificial and substitutionary death of our Lord, the Lamb of God, on behalf of a world of sinners
the resurrection of Jesus from the dead, his ascension to the Father's right hand in glory, and his coming again for his bride, the holy catholic church
salvation by God's grace through faith alone, which begins with personal regeneration by the Holy Spirit and culminates with the resurrection of the body and eternal life in the presence of God and the holy angels in a kingdom without end


I have undoubtedly omitted--unintentionally for the most part--some crucial doctrines and teachings. One in particular concerns the nature and purpose of the holy Scriptures. Suffice it to say that the vast majority of true Christians believe the Bible was supernaturally inspired by the Holy Spirit, and in its entirety is sufficient for all matters of both faith and practice. 

Granted, how various Christian groups interpret the Scriptures is a nettlesome issue, especially for those whose minds are made up. One of the marks of spiritual maturity, however, is the ability to agree to disagree agreeably with those who "rightly divide the word of truth" just a little bit differently than we do, particularly on what could be called "grey issues" or issues of lesser importance than the non-negotiable ones I've listed above. A very short list of these could include


water baptism, particularly when, where, how, why, and by whom
the Lord's Supper (aka communion, the Eucharist, breaking of bread), particularly when, where, how, why, and by whom 
the number of sacraments and their relative significance
church polity, or governance, whether of a single, independent church or an entire denomination composed of thousands of local assemblies of Christians worldwide
behaviors which are sometimes proscribed by a given church or churches but which are not explicitly dealt with in Scripture, behaviors which could potentially be stumbling blocks to a "weaker" Christian, but could also be engaged in by "stronger" Christians who have the freedom of conscience before the Lord to do so


In conclusion, tradition plays an important part in the ongoing life of the local church. Determining what traditions should be kept and guarded and what traditions should be replaced when they outwear their usefulness is seldom an easy task and balancing act. To avoid unnecessary schisms, perhaps all Christians, at least once a day for the rest of their lives should recite those famous words from a 17th century tract on Christian unity written by Rupertus Meldenius during the Thirty Years War:


  "In essentials, unity; in non-essentials, liberty; in all things, charity."