Every Christian is in full time Christian service, always on the front line. Even a Christian who doesn't want to fight is in the conflict. A soldier may be untrained or in poor condition and drop beside the road before the enemy is reached. Or he may be killed in the first skirmish. The primary thrust of evangelism is through the individual.
Individual believers circulate throughout the community in various functions.
The principle or witnessing for Christ is that you do it in your own environment. Preparation implies the ability to fight. Preparation means flexibility and the ability to use God's Word in witnessing. You need to know about Reconciliation, Propitiation, Redemption, Imputation, Regeneration, and The Barrier, as a starter. You don't have your shoes on your feet without this preparation.